For some reason, I didn't find the LTX on tirerack.com either. They do appear on Costco's site, but you have to dig deep to find it. I bought the M/S's, as I spend a lot of time on pavement, and they work extremely well.
It's your call on the size difference. I also replaced the spare, even though both were the same size. I figure that the spare should be of at least the same quality as the rest in case I need to drive a long way on it. I justified the extra cost by doing 5-wheel rotations, which makes everything last longer. When I replace next time, I'll leave one worn (yet servicible) tire on as the spare.
It's your call on the odd-sized spare dilemma. I wouldn't do it. You can't use PT 4WD with odd-sized tires or you will destroy the transfer case. It will cause the axle clutch packs on limited-slip axles to overheat and wear quickly. The vehicle will handle badly. There are no positives to keeping an odd-sized spare other than money, and that's not good enough in my book. Justify the extra cost by doing 5-wheel tire rotations.