I've used/use both Firewire and USB interfaces. Over time, USB's limitations became less of a problem, and Firewire's advantages became less relevant. Both can be touchy, for different reasons, so Jim's advice is well-taken.
However, regardless of theoretical performance issues, there are also practical considerations. Frankly, I think the issue of computer ports and audio is a mess. Firewire is fading; laptops often don't include Firewire any more. You're okay if there's Thunderbolt, but now you're dealing with TB-to-FW adapters. USB 2.0 is fine, but can be touchy (separate PCI card with ports has been my solution) and 3.0 has a lot of complexities - check out the RME web site for the hoops you have to jump through for reliable USB 3.0 operation. Thunderbolt looks promising, but is costly and hasn't really gained traction.
USB interfaces outsell Firewire ones by a wide margin, similarly to how VHS outsold Beta and we know how that ended up. So I think the odds of having an interface that works with more computers goes to USB. But, Firewire isn't going away any time soon so if your computer supports it or you have a Thunderbolt interface, in theory a Firewire interface won't become a doorstop for quite some time.
So really, I think the bottom line is they both can work fine, but whether they do or not depends on your specific computer.