I use a Profile rail mount on my Tri/TT bike and haven't had any problems with it. The cages are very tight. I can't imagine a bottle falling out of them. But, like John said, they're removable so you can replace them with metal or carbon cages if you feel the need.

EDIT: I'd get the post mount if it will work for you. There is a pivot to adjust the angle on the rail mount that did slip on me once. It hasn't been a problem since I cranked it down, but I think the post mount would be a tiny bit more secure since it doesn't appear to have that pivot. Unfortunately the post mount wouldn't work with my aero post.