USB2 is slower than firewire in sustained transfers. That said, either should (in theory) exceed the 16x data rate. Needless to say, YMMV. There are many poor USB2 controllers around that struggle to make it to 8x speeds. But unless you have a burner that's modded for removing rip locks, you'll likely not see any issues. Most drives rip video at 8x or 12x. Some will hit 16x on single layer only.
The other common issue with USB is that it does not provide a sustained transfer, as USB is a one-way connection. Results in a lot of little blips and burps in the data stream that can trip the burner to slow down. Firefire does not suffer from that. But again, odds are fair that USB will be fine.
My advice is always to get a dual-link case that allows you to go either way.