As kids we would jump feet first from a 45 foot cliff and by bending at the waist as we entered the water, we would not go 10 under.
We could turn so quick that our legs would sometimes shoot out of the water a foot or so from our momentum.
Oh, I was over 6 feet tall in the 6th grade so I was not exactly little.
Gelatin, when made up to dessert recipe strength, only sets very tenuously - stirring it a little effectively reduces it to a fluid again.
That said, I would say that a person jumping into water is probably always going to be safer, just because whatever happens, it will more closely match their expectations - any variation from normal water, even if it’s technically trivial, is a potential risk factor.
I jumped off a 12 ft roof into an 8ft pool without turning my body and almost broke my neck and knocked myself out. I have more respect for heights now when diving.
I know they are professional divers, but there’s a place called Dutch Wonderland in Pennsylvania that has a diving show. When I was there just a couple of weeks ago they were diving from a max height of 70 feet in to a 9 foot deep pool. So, yes it’s quite possible to fall in to a pool from that height and a 10 foot pool and have no problems, at least if you know what you’re doing.
Is it the same volume, though? That is the salient point.
I get what you’re saying, but for anyone who didn’t know, they do make pineapple flavored jello.
And you can even put real pineapple chunks in jello as long as they’ve been heated enough to inactivate the enzymes that would otherwise prevent the jello from setting up.
Depending on how much liquid was in it, I might not even break the surface; it might have a trampoline effect. And if I did sink in, maybe I could eat my way out. 
ISTM that a key issue is whether you need the substance in the pool to be more viscous and dense in order to prevent yourself from hitting the bottom, or less viscous and dense so you don’t die from hitting the substance.
According to this siteJello ranges in density from 0.98 to 1.3 grams per cubic centimetre, which is to say it ranges from about the same to 30% more dense than water.
ISTM for a pool that is so shallow that you would otherwise sustain injury through hitting the bottom of it, dense Jello may be better because it would be better to be slowed by dense jelly than entirely unforgiving concrete. But if the pool is deep enough that water will stop you before injuring yourself on the bottom, then you are better off with water because it will decelerate you more gradually then Jello.
Close enough. Serving Size 2 cups of jello takes 2 cups of water to make according to the “standard” (non jigglers, non-mouse, non-quick-set) instructions. I’m sure there’s a little rounding going on, but it’s pretty close to a wash. Which makes me suspect that the answer to the OP’s question is going to be “doesn’t matter” as well.
How did you jump & dive at the same time? No wonder you hit the bottom… 
Even if Jell-O is as uncompressible as water, the viscosity is more the issue with the OP’s question.
As others have pointed out, it’s pretty easy to limit the depth you travel when jumping from a height if you know what you’re doing. If you do things right, at worst you’ll just bump against the bottom at a very slow speed.
Jello is going to hurt a whole lot more.
If you bellyflop it would probably still be worse in Jello but both will hurt. I suspect your risk of neck/back injuries would be much higher will Jello than water.
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It sounds like there’s no reason to believe that gelatin (e.g. Jell-O) would be any more compressible than water. So if it’s equally non-compressible and it’s more dense, then it’s probably going to be worse for you.
Argument for Jello:
it’s going to slow you down more (as you slide through it) before you hit the hard bottom of the pool.
Argument against jello:
the initial impact (with the jello’s surface) will be much harder (than the waters surface).
What we need here is a fluid that gets more dense as you go down. Can this be done with gelatin?
There was a previous thread (a few years ago, now) about the ideal material for falling into. As I said then, I think that the material is less important than the engineering. Falling into a steel plate might suck, but thin sheets of steel wool spaced apart from one another, increasing in thickness, might not be too bad.
Likewise, if you could aerate your gelatin, possibly you could lower its density and give it greater compressibility. Once you can do that, layering it into varying densities should be relatively easy.
How about not just falling? Works best IMO.
Define falling: Landing on your back/face with one bent leg & an arm under you? ( Uncontrolled fall? )
A purposeful dive or jump feet first? Someone who knows how to do it?
Age, physical condition?
IMO, if a large & deep pool of Jello, You would not be able to get to the surface without help unless you land flat. Then it won’t matter cause you will be dead.
What about the guy doing a belly flop from 30 feet into an 8" kiddie pool? IIRC, those are the numbers but I could be off a bit. Apparently he does this a lot and is all rigged up to do it.
Are these Jello shots?
Same here, although in our case it was probably 40’ high with a 15’ (natural) pool at the bottom - of a rope swing or “free-jumping.” Even from that height, you could, if you splayed your arms and legs properly, land without submerging (or nearly not - tough to tell with that splash). But you certainly weren’t going to the bottom.
That said, if you didn’t do everything right, you were definitely getting banged up on rocks. In retrospect, we were lucky none of us got more than some bumps and bruises (and the occasional busted bone).
Surface effects (incl. surface tension) are often mentioned in discussions about falling into water, but they are a very trivial factor - surface tension is tiny - the tensile strength of jello is tiny.
The problem with impacting water is that it is dense - accelerating it out of the way so that you can displace it takes considerable force - and when this needs to happen over a very short timescale, it’s a bad thing.