Which is better? That depends on a lot of factors.
Both FireWire (400 & 800) and USB (2.0 and beyond) have plenty of bandwidth available for multi-track audio recording, even at high sample rates and bit depth.
Latency has much to do with the drivers, the host computer, and the audio interface used.
Now, I've seen someone else talk a bit about throughput jitter. (This has nothing to do with clock jitter on the interface. Don't confuse the two.) Basically, FireWire can operate in a mode where there is a steady stream of data sent from a device to the host computer.
With USB, data can also be sent in a fairly steady stream of data, but if another USB device on the same root hub (external hard drive for example) wants a ton of bandwidth, your audio interface shares what is available. With FireWire, devices also share the available bandwidth when chained together... it is just less common to have more devices.
This is not to say that USB is inferior in this area. Simply plug your audio interface into a root hub by itself, or with low-bandwidth devices like your keyboard/mouse. It's really that simple. Don't have one available? Buy a $10 card and plug it in. Chances are you'd have to do the same for FireWire anyway, but that FireWire card will cost you $80.
Universality (Is that a word?)
How many computers made in the last decade (nearly?) have USB 2.0 interfaces? You would be hard pressed to find one that didn't.
How many with FireWire? Many for sure, and you can always buy a FireWire interface for your computer, Mac or PC, desktop or laptop (assuming available expansion slot). But if you drop your laptop on the way to a gig and need to borrow someone elses', you might not be so lucky to find that FireWire interface. Whether this matters or not is up to you. It may not matter one bit depending on your situation.
You should choose an audio interface based on whether or not it meets your needs, and not on how it attaches to your computer. While FireWire at a low level has some strengths for dedicated bandwidth, it matters very little (none for many) these days.
You will always find people that say they used to use USB Audio Card X, and upgraded to FireWire Audio Card Y and noticed lower latency. Well of course, they probably bought something a lot newer or better. Either way, they are comparing apples and oranges.
Here's a personal experience for you... on my rig at home, I have a Tascam US-1641 USB audio interface, and my input latency is under 3ms! I haven't even optimized anything. It's even plugged into a chained 7-port hub that is loaded up with a webcam, another audio interface, an external hard drive, and some other crap. It's quite reliable. But again, I'd urge you to look past personal experiences here and there and read through my points above.
Choose an audio interface on its quality and features. If you find something you like that uses FireWire, then go that route. If you find something you like that is USB, go that route. Don't ignore one or the other simply by how it attaches to your PC. The reality is that doesn't matter anymore.