If kept seperate? Probably Scotty - experience and training has to count for something, even against a raw talent like Miss Frye.
But working alongside each other, Scotty wouldn’t be able to keep his eyes off her and Kaylee… besides the fact that she likes her men younger and taller, we all know that she’s capable of diagnoising mechanical problems mid-coitus. Nothing can distract her from her engines.
So did Scotty in his later years.
How many parts of Serenity would explode with 215 gigatons of explosive force just by turning them off? How many teleporters does it operate (let alone ones that don’t just materialize a copy of the subject you’re trying to transport, while destroying the original)? Does it have faster than light engines, or use engine plasma instead of electrical wiring?
As much as I hate to say this, the Enterprise (even with no bloody -A, -B, -C, -D, or -E) is a vastly more powerful and advanced ship, requiring far more scientific and technical experience to handle. Kaylee—for all her skills, and I’m sure she’d do just fine if given the chance to train into the big leagues—is a mechanic. Scotty is an engineer.
Yeah, but Kaylee’s cute. And she devours strawberries with passion.
Wash probably had at least some notion of what part went where, since he does some maintenance (and jury rigs the transponder in Out of fuel). That said, I doubt he could make sense of what mods and duct tape wonders Kaylee applied to the engine itself.
Scotty, for all the reasons stated. Plus he can invent transparent aluminum and come up with a method for emergency teleportation at interstellar distances.
Kaylee.
Scotty was brilliant, don’t get me wrong. But the guy was working on a ship that, 99% of the time, was in perfect working order. Pristine, just out of some space port or other, fresh oil changes and air freshener on the rearview mirror. Kaylee, however, took a ship that was literally junked and got it running. Then, on $0, kept it running no matter what, aside from that one time which we all know was in no way her fault.
Put Kaylee on the Enterprise, and she sips mai tais and eats strawberries. Put Scotty on Serenity and in a week it falls out of the sky.
They also show him on-screen working with Kaylee trying to repair Saffron’s sabotage in Our Mrs. Reynolds.
… and falls in love with Spock.
Scotty. At the time of TOS, Scotty is the Chief Engineer of one of the most advanced craft that Starfleet had. He has helped design (and later redesign) the systems that run a craft meant to go for years with resupply, handle situations ranging from humanitarian to espionage, to pure military. He has shown the ability to adapt to ancient technology almost instantly. He understands not just how the Enterprise works, but why it works that way. He’s one of the top engineers produced by a civilization whose technology has eliminated the very poverty which causes Kaylee so much her problems.
Kaylee is a gifted mechanic, but lacks the underlying theory. Her responsibility is for a craft that never leaves its own solar system, and is rarely more than a few hours from help. OTOH, I’d rather go drinking with Kaylee, she’s cuter, and wouldn’t drink me under the table nearly so easily. 
They’d go binge drinking and have sex.
What, we aren’t writing slash fiction?
I’m so embarrassed.
Heck no. It was the first mechanic who did that. They went off a ways and then got stuck when she came along. And Kaylee doesn’t deal with major combat damage on a weekly basism, whereas Scotty has to sometimes keep things going while large holes are being punched into his ship.
This made me laugh so hard I had a coughing fit.
Scotty, by a light year. The mechanic/engineer comparison was apt.
Scotty - 66 episodes, 7 feature films, 22 animated episodes, at least four video games, and a theme park ride. Household name, numerous contributions to popular slang, international reknown.
Kaylee - 14 episodes, 1 feature film.
As for in-world comparisons, I find the mechanic/engineer comparison to be the best thus far.
Consider Scotty’s feat with designing and constructing, without a large crew or spaceport, a tank which could hold, and sustain two humpback whales, within a ship designed by another race and which he had very little experience with. The calculations necessary to build a structure which could support the shifting water would be extremely daunting in the best of situations. Working in Klingon on top of everything else puts him in that 0.00000001% bracket.
Kaylee is never shown to perform anything like this feat of engineering. The closest may be when she and Wash restore the ambulance vehicle from Ariel, but that was repairing something, not creating something new. Figuring out how to reprogram the trash collector bot would be her second best effort.
Scotty was a handsome man with good people skills and was a leader both of his crew of numerous engineers and of the technical field. In his TNG appearance Geordi quotes some specs for impulse engines and Scotty says to ignore it, he wrote it and he left himself some wiggle room.
Kaylee is a cute woman with reasonably good people skills. She was never shown in a position of leadership or considered an authority on a subject by any audience wider than the Firefly crew.
Scotty understood the principles of what he was working on and could hold his own in theoretical conversations with Spock. He understood the subjects well enough to fabricate and implement numerous devices from these theoretical conversations.
Kaylee never shows these kinds of skills.
Enjoy,
Steven
Physical fight: Scotty. Kaylee is scrappy but not big enough, and probably hasn’t had any martial arts training, which I think Starfleet officers receive.
In a race to fix their own ships, solo, after being damaged to an equivalent degree, that degree being measured in Non-Genius-Engineer-Man-Hours: Kaylee. Not only is she quite skilled, she’s working on a smaller ship with a more compact engine area. Even though I love Scotty, he’s got a much bigger ship and the areas of concern may be widely far apart. Without assistants to do his bidding, Scotty would lose a lot of time going from place to place.
Unless we’re talking about Mythbuster’s Scotty, it wouldn’t be slash, it would be het.
Therefore he is the most realistic engineer in TV history.
And he could command the ship in battle.
True, but it’s like saying that because I do a good job keeping my 18-foot fishing boat in order without support from anyone, I’m as good an engineer as the chief engineer aboard USS Nimitz.
Scotty could do Kaylee’s job; keep a small ship running.
There’s no evidence Kaylee could do Scotty’s job - command a team of scores of of people running a machine a hundred times more complicated. Everyone who hasn’t been a manager thinks it’s easy. It’s not. Kaylee would be a member of Scotty’s crew, and would be at least ten years away from working her way up to an equivalent post.
If Kaylee ever found herself on a ship which used plasma conduits, the first thing she’d do would be to rip them all out and replace them all with good old fashioned solid wiring. Advantage: Kaylee.
Y’know…I think they’d get along famously.
Mind you, pity the poor sod they ganged up on…can you imagine the team of Scotty and Kaylee ripping Mal a new one for not getting the parts that were needed?
shudder I’d rather take on all the orcs of Space Sauron, boarding from the Death Star…
Lets see you stick your hand into a runing engine that’s going to explode while the ship’s computer counts backwards from … 10 … 9 … I need those engines Mr Scott … 8 … **I’m doin the Best I can **… 7 … Mr. Scott … 6 … It’s no good, we’ve only got six more seconds (minus this lengthy conversation) … 5 … Mr. Scott, I’m well aware of the time, you must reverse the polarity on your phase incriminator … 4 … I can-note do it, you must to eject the core … 3 … MR SCOTT … 2 … 1 … late night dating commercial.