I've got the 500 ml because it was on sale and a water bottle. DO NOT GET IT, there's no place to hold on to it. During the winter I almost drop it every time because i'm wearing gloves. During the summer it's just annoying because there's no place to grip it, the 750 gives you a little ridge to hold onto, but from now on i'm making sure my water bottles have a good spot to pull them from a tight cage.

I strongly advise against the 500 ml, but then again, it's just a big inconvenience, it still holds water