If these were my only choices I would say a kisslock. However I much rather have a zipper closure for safe keeping. I'm not a fan of easy access to my bags because that means others will have easy access.
envelope style for me, i find them easier to stick under the arm when you need two free hands, the kisslock is a bit uncomfortable in that position, and as someone else mentioned they can break a nail - aaarrrrgggghhhh!!