I find them fulfilling incredibly different roles in the decks they end up in. Sylvan Library is just better card selection off your draw every turn for slower decks where you can afford to take an early turn off, while Leap is usually just used to turn bodies into more bodies and accrue additional advantage for aggressive decks. If you need to keep applying pressure with your deck, then Leap is more effective by refueling you with threats. If you have any tokens, Leap is pretty incredible for card advantage by turning chump blocks into cards.
If I was forced to choose one, I'd keep the Library because it's stronger in more cases and works well with fetches and any library searching effects. I would just keep both of them though, I'm all for more noncreature permanents in green that impact the game.
Oh, totally ok with them being different role players, I wasn't looking for the same effect, I'm fighting more over the slot. Im all for more non creature effects in green too, but Im having a hard time finding more space, so it is what it is. The todbit I like most from what you are saying is:
Sylvan Library = better slow
Evolutionary Leap = better fast
Generally speaking of course. With what I'm trying to do with my green section, I've been thinking recently that Leap is operating at the wrong "speed" for what I want to be doing. And if I want green to be featured in more decks with more noncreature spells than might be usual, maybe library is the better choice in that slot for me, based on this pacing info.
Grillo made a deck recently that your post reminded me of. A deck featuring green, but definitely better suited to Library, especially with looting effects, than it is to Leap, with its slightly low creature count.