Overall, which gender of Panthera Leo is more aggressive, if such a generalization can be accurately made?
The male’s mane job is to defend the pride, so it’s the male. And it’s Panthera leo. 
Although the OP does beg the question - aggressive to whom?
If you’re a human planning to walk up to a lion and poke it with a stick, you may elicit a more robust response from a male, for the reasons JM describes.
On the other hand, the females do almost all of the hunting for a pride, so if you’re a prey animal I suppose you might consider the females more aggressive, if by aggressive you mean “likely to rip your throat out and eat you when you’re minding your own business”.
On the third hand, if you’re a lion cub, you need to be extremely cautious around unrelated adult males.
The male won’t start trouble, only finish it. If there’s no hyenas attacking, he’d rather just lie around and sleep all day.
On a slight tangent, I just learned that lions were among the megafauna found in N America until about 10,000 years ago. Not mountain lions - more like modern African lions, and 25% bigger.
I’m the only one who read the title as “male or female loins” aren’t I.
Never mind. I reconsidered my answer.
Typical cat.
The lion once had one of the largest ranges of any mammal, extending from Africa across Eurasia through North America. (There is some debate whether the extinct American Lion was a separate species or just a subspecies of the modern Lion.)
No simple answer here.
MOST lions of both genders, sleep 20+ hours a day. So, most of the time, neither lion nor lioness is “aggressive.”
There’s usually just one alpha male lion in a pride. By definition, he is CAPABLE of being a real badass (he has either killed or driven away every other male lion around). But when he’s not being challenged, he isn’t necessarily aggressive. As others have noted, one of the perks of his position is that the females do the hunting for him. So, when he’s not fighting off challengers, he CAN be lazy if he chooses.
Young males are usually driven away once they reach an age when they could pose a threat to Dad. Those young males will wander for a few years, doing their own hunting. Those young males can be VERY aggressive and dangerous, to humans and to herbivores. Their hope, of course, is that down the road, they’ll get bigger and stronger and eventually stumble on a new pride with an older alpha male ready be be challenged.
If two young brother/cousin male cubs get driven from the pride at the same time, they may stay together during their bachelor years. And it’s not unknown for a pair of young males who’ve bonded to challenge an older male together, and then rule over a harem of females together.
Actually multiple sibling/cousin males ( called a coalition ) to a pride is the rule, not the exception. Which when you think about it makes sense. Not only do related males share genes that they can pass along, but multiple males are far more capable of driving off other males, asserting control and maintaining that control longer before they wear out. In fact the need for multiple males to seize and hold a priode is so strong it goes beyond relatedness - unrelated males will also occasionally hook-up to form coalitions as well.