Firewire 400 is only slightly faster than USB2. In fact USB2 is 480mbps on paper, vs 400mbps for Firewire, but Firewire is more efficient so the real-world speed is slightly higher.
LAN would be fastest, but you need to get a NAS fast enough to actually push data to you at gigabit speeds (most of them are CPU-limited to much slower speeds).
Good NASes can get expensive, to the extent that I gave up and bought an old tower PC for 90 quid (quad-core Opteron!) and put 6 hard drives and a second gigabit network card in it. With Windows Server 2012 R2 NIC teaming and RAID6, voila: super speedy reliable access (but maybe a bit overkill for your needs).