Get a 360 then get mw3... gamers on xbox are considerably better then ps3 players for some reason..everyone on xbox is a tryhard. So if you want more challenge, go with xbox.
I would start off with MW3. Because it will be a new game for everyone. So you have a better chance at starting off even with the rest of ps3 players. Just dont expect to have some crazy high K/D ratio.
It's all personal choice, there's no 'PS3 is better than XBOX' or vice versa. Each console has their own benefits and disadvantages. Most people these days just buy whatever console their friends have. Ask your friends for advice and see what they say if you're still undecided.