Assuming a system is properly configured, FireWire 800 should provide the better read/write performance.
Firewire is much faster than USB 2.0!
It’s slightly better, but since it’s at least very hard to get it stable (connection problems and not 100% good behaviour when dis/connecting in Win XP SP3, I cannot recommend anyone to use it on Win XP. Not worth the trouble. The Unibrain FW800 drivers are OK, but if you have multiple LUNs (as is commonly the case in Win XP since it has the size limitation of 2TB) this doesn’t really work well if you want to be able to disconnect & reconnect the Drobo now and then without rebooting the PC. If you also happend to have other FW800 device(s) which work better with MS Firewire drivers, well… In my experience the overall average Drobo v2 throughput with USB2.0 is almost as good in Win XP SP3 as FW800 is, and the USB host connection is much more stable and trustworthy.
There was a substantial amount of information about the performance of Drobo v2 Firewire 800 vs USB2 and what Mac OS X users had measured and tested in the old Forum threads which are now gone. The conclusion was pretty much that the results in the graph shown on the Drobo v2 product page “the Drobo with FireWire 800 and USB 2.0 Performance Charts” may be acheived under some circumstances (such as a completely empty Drobo) but very (most?) often the throughput will be close to the red bars in the picture, not the blue. This is one of the major reasons Firewire 800 host connection isn’t worth the effort under MS Windows.
Remember that a USB 2.0 bus will run as slow as it’s slowest connection, in other words, if you plug in a USB 1.0 device, everything slows down to that speed. This means you need to connect your drobo to a USB port that it will not be sharing with other devices you or others may later add to achieve that top speed consistently.