get the marble, small size and look better, make sure get a group of them, they will feel more secure and active.
No doubt, I plan to get 6-8 or so... My GF really wants the silver ones, and although I like the marble, she lets me do whatever I want with the tanks in our place so I'm going to get whatever she wants in this case. She had hatchets as a kid and gets super excited every time she sees them at the LFS. Plus most everything I've read lists the marble and the levis at the same max length of just under 1.5".
have to let you know they fly, if you don't cover your fish tank, your cat will be happy.
I definitely have heard some pretty amazing stories about their leaping ability. Tank is already covered so its ready to go for them.