Go Cat
In those years, for a guy who cares about dependability, definitely go Cat. Polaris was lost in those years IMO while Cat was king of the hill.
I had a 2011 M8 that was great. It did need primary clutch work awfully early though, but go check out Dale at Cutlers and he will have a solution for you there.
Depending on exactly how important reliability is vs. performance, you could maybe consider an M8 little brother, the M6. Likely to be more reliable, and at this point in the game, likely to have been ridden by a rider that is a lot nicer to it than the 800. I do tend to agree with Jay Nelson that it's a little tough to put dependability and a 9 year old 800 sled in the same sentence. Likely to have better luck with a 600.
Don't get a Nytro though. The stock ones have held their value really well because they are 4 stroke, so you won't actually get one cheap like joshkoltes suggests, and while they are very dependable, it's just too much sacrifice to make for all that weight. Unless dependability is #1 and #2 is a really distant second - then maybe get a Nytro.