First of all, I haven't tried MW2 on the PC, so this is from experience on the Xbox:
I prefer matchmaking to Dedicated servers. It's just so much quicker and easier. You just start the game, it finds players in the same skill range within the same-ish area, you have 15-20 minutes of good fun, and then you refresh for a quick new match, with a refreshing new set of players.
With dedicated servers, it takes me quite some time to find a server that has quite a lot of players without being full, and I'm oh-so-sick of annoying server side mods, stupid server messages, annoying admins and server lag, as well as server "regulars" thinking they're all leet and own the server themselves. Also, jumping in in the middle of a round is never really fun.
I dunno, I just vastly prefer matchmaking, dedicated servers stress me out.