Just a few more thoughts on both skills.....
The jump itself has to be done in good posture because that's a major contributor to straight take offs and balanced landings. The problem presented with this good posture is that a kid can't look down to see where they're landing, and haven't the experience of thousands to give them the confidence the beam will be exactly where they left it when it's time to land.
If you ask me, I think kids prefer the split leap because they have a better sense of having gone up straight when they only have one leg to keep track of during the releve' into the take off. The forward motion also gives them a sense of having taken of straight and centered over the beam. The last advantage is when doing a split leap the run into the plie' rocks them into a forward leaning position they can use to cheat a peek at the beam, and then the back leg lifting behind them demands they pull their shoulders back..... viola, you get good posture anther having peeked at the beam during the take off.
The reality is if a kid disciplines herself to keep good posture with a tightened core that stays centered over the beam, the split jump is easier to get done because kicking/lifting the back leg will nearly lift the front leg to it's 180 position, and that motion can start as soon as their foot leaves the beam.
Sure, they can kick/lift their back leg on a split leap, but they can't really catch that back leg up with the front leg unless they hold their front leg in position at the apex of the jump to allow their back leg to catch up, and that takes a ton of practice on the timing of the front leg lifting and stopping as well as the back leg finishing the push off in a posture and tension that promotes getting that leg to lift. Failure to time and posture the back leg is easily seen in beginners who have their front leg level with the beam and haven't left the beam even though their hips are a foot or more past their back foot, which drags along behind them the rest of the way.
I just don't see all that happening in a split jump, but for some reason there are kids who prefer them over split jumps.