Choose the software ecosystem first - sounds like Android is your choice here. (I'd be sorely tempted by the newest Kindle Fires if they weren't so locked down and hacked ROMs weren't so sketchy, the hardware is pretty sweet aside from the mediocre CPU.)
Choose size - 7" is nice for the portability, but maybe not so much different from a larger 4.5"+ smartphone if you have one. 10" is great for home use, still reasonably portable though, but may get tiring for one-handed use. I think the display size jump of a 10" tablet is worth it if you have a smartphone already though and don't see yourself traveling with the tablet a ton.
As for specific device, either a Nexus for best OS updates and rooting, or at least something that's widely supported and generally open to modding like a Samsung. Being locked out of OS updates, or relying on sketchy mods that only work half-ass isn't great. Anything I'd buy today would have to at *least* have Android 4.2 when purchased - no '4.2 coming sometime!' b.s.
I've been tempted to get the Nexus 7 a few times, even though it's not much bigger than a smartphone it's cheap
but once I saw how poor the WiFi performance is (see Anandtech review) I decided to hold off until the next iteration at least. I'd really like to see Google make the next small tablet screen size closer to 8" though, because that would give a more notable size advantage over a smartphone while retaining the compact size. (Yes, that's something Apple did right, Amazon too with the Fire HD as their 'large' tablet at 8.9".)
A final wildcard is Windows 8 x86 tablets. They aren't great right now, but I expect that the updated Atom for inexpensive tablets will be pretty nice, and if you want to spend $$ Haswell tablets will improve battery life significantly, and if AMD can get any decent design wins their new low-power chips might be good. Unless you're Linux-only you're already in the Windows x86 ecosystem from a desktop or laptop. The question then becomes, do you want your tablet to be like a big smartphone, or like a portable PC. Tough choice honestly.