FYI typical speeds on USB2 vary considerably depending on the hardware. Speeds of 12, 18, 26, 36, and 38 MB/sec are strangely common, I assume these clustering of datapoints is due to a small number of different USB chip manufacturers with their own distinct performances.
The best bridge chips nowadatys can do at least 36MB/sec. FW400 usually does 32 or 39 MB/sec, again depending on controller. So FW will usually win.
I don't have the supply of hardware needed to test USB3 speeds yet, but they appeared to be maxing the drive I was testing with. (as did my tests with expresscard esata)
26MB/sec is the bottom number I see on macs. (with the left port of the older 15" mbp being the exception) The 12 an d 18's are usually on those cheap usb-to-sata/ide adapters. Don't use those adapters to connect to your burner and you should be ok with burning via USB.
