Does your Sony have both connection? If so, that would be strange. If it does, I would go with USB because the transfer rate should be faster (that's if you compare USB2.0 vs Firewire).
I have a Canon HV10 HD camcorder that uses Firewire. When I transfer there's some skipping, pixelation, pops, during capture. When I do the same with my Canon HF20 (USB 2.0), the capture is very smooth.
Intel i930 (4x 2.80ghz), 12 gb Corsair Dominator DDR3 ram, 1.5gb Nvidia GTX480, Asus P6t mb, 850w XFX PSu, Windows 7 HP