The Tundra has always seemed a little large for my needs, but they are very nice trucks. I have a 2008 Tacoma Double Cab with the short bed. The bed is short for sleeping, obviously, but fine for everything else, even hauling plywood so long as everything is tied down. You could go for the long bed version, but then you're getting into quite a long vehicle, which may or may not be a big deal depending on where you plan to go. I prefer the slightly less overall length of the short bed.
As for the rear seats in the Tacoma, nearly everyone who has sat back there has complained that the seats get uncomfortable fairly quickly. It's not so much that they are cramped as much as the angle of the seats and the seat backs. At least that's what I've heard. I never sit back there, so I can't offer anything beyond that. I have no info on whether a Tundra's rear seats are better, but with more room I expect people would find them more comfortable.
The TRD Offroad package in 2008 came with a locking rear diff and an "offroad" suspension that basically consisted of some Bilstein shocks with the regular springs. I'm not sure what's on offer currently. The diff is great, the suspension was a dud and not worth the extra cost. It had very limited travel and bottomed out on speedbumps. If you put two kids and some gear in there, you'll be a lowrider for sure. I quickly ditched the factory suspension and upgraded to an Old Man Emu suspension from ARB, which was a vast improvement. So the only thing I ended up keeping from the Offroad package was the rear locking diff. If I had to do it over again, I would have bought a regular model without the Offroad package, and installed an ARB airlocker rear diff and also the same suspension I ended up adding. It would have been a little cheaper, and the ARB airlocker is better than the Toyota unit, in my opinion. It's not as fussy to engage, for one thing, and I had one in my old FJ40 Landcruiser and loved it. I'm fine with the Toyota locker in my Tacoma and don't plan to replace it, but like I said, if I had to do it over again...