You aren't really running a native install of the OS, you are running a virtualized sort of system. I did this with a first gen Xoom and it was very slow, but then again that was before tablets had Tegra chips with 4 cores. I'm sure whatever tablet you choose will end up being similar just keep in mind you should have a micro-sd card slot. Also note that running Backtrack or any other OS on a tablet is neat and educational, but not really practical. I imagine we are a couple of years away from this being a useful exercise, for now as I said, its mostly educational.
I should follow that up by saying there are some interesting attack vectors that mobile devices with pentesting software on them can be used for. I have heard and seen devices being sent in packages to companies as fake deliveries. They show up, the receptionist signs for it and the individual to which the package is addressed says "I never ordered that", so the receptionist arranges a return for the following day or whenever. In the meantime the attacker uses the device to scan the networks, possibly getting into company guest networks or maybe breaking some poorly secured wireless network depending. This is relying on a lot of different stars aligning, but I have come to learn they frequently do. I would still say unless that is something you are planning on doing, and obviously for a legal test, I would stick with a laptop. Tablets are somewhat of an expensive ereader/browser (perhaps the Google Nexus tab excluded).