Chrome is bloody, bloody fast. Everything happens pretty much instantly, whereas Firefox still has a perceptible delay, small as it may be. Firefox has addons. Except Chrome does too. But Firefox's ones are generally more mature. Though Firefox seems to like to break compatibility with them every time you update to a new version. Firefox has a lot more tools; you can install different toolbars, etc, and you generally get a lot more choice over how it looks. But Chrome is really, really simple, and that means lots of screen space too. Firefox works better with forums. Chrome works much better with fewer tabs open; everything starts slowing down after about the 10th tab. Whereas Firefox is a lot slower overall, but its speed doesn't degrade as much when you open lots of tabs. Chrome is practically multithreaded - even though each thread can only run on one core as far as I can tell, Chrome creates one for every single tab. On the other hand, this also means that if you have lots of tabs open when you call up the Task Manager you can't see anything useful because the whole processes pane is taken up with lots of Chrome.exe's.
tl;dr: It depends on what you like. Use both of them.