They're not all the same!
Though thank you for not including the abomination that is IE 6.
As time goes on they get closer and closer to being all the same because the best ones (such as Chrome and FireFox) are open-source, so everyone just steals all their shit anyway.
The differentiation is very small.
I use Chrome because (quite flatly) it is the closest to properly running HTML3 and HTML4 code of all the available browsers (save Opera, but I hate the interface of Opera), so it gives me the least trouble.
Also the fact that each tab is its own process, so if I have a tab crash I just lose the one tab and not all of them.
And also the fact that it's incredibly secure because it offers no low-level access to anything that runs on it, save plugins, except I don't even use Flash anymore because HTML4 streams video and audio better than Flash.
I could go on.