When you're talking about BT vs USB it's usually more a question of costs and convenience than power consumption. BT keyboards generally speaking will cost more money, but give you that "wireless freedom" so to speak; USB on the other hand tends to be cheaper, and doesn't require your device to have it's own power supply, so generally costs less. In terms of power consumption I would hazard that you're probably pretty close (so long as you have a pretty standard USB keyboard and not a crazy high-current one) with BT probably at a slight (not sure if noticable) advantage. I've got a roll-up USB that I picked up for $7 that I've used on the tablet for about 2 hours without seeing any noticeable difference in battery drain; when I checked dmesg though I noticed that the kernel told me the keyboard was only asking for 23ma of power though, so that's got a pretty big impact on it.
I've also got a BT keyboard but I've only ever used it for an hour or so continuously, and again, no really noticeable difference between that and just having BT turned off. My understanding is that BT has been tweaked and optimized in the same way Wifi has, so leaving it on and connected has almost no major impact on battery life unless you're constantly sending/receiving data (i.e. your cat falls asleep on the keys and fills your screen with "FSAFDA...").
In either case the thing to keep in mind is that while the BT might seem like it lasts a little longer since it has it's own power supply, it also can be more of a pain, especially when travelling: 1) Requires a second charger, and 2) Can't use it on a plane. But again it depends what you're primary uses for it are and what you're really after.

Thanks,
-Rob