I own all consoles and barely play the 360, it's the glossy 250GB slim. I bought this used as I was extremely pissed off with the lacklustre build quality of the launch 360's (I bought two and RROD occured on both).
I would say go for a PS3 since it has Blu Ray, free online, PS Plus and better exclusives than 360 it will be a better future purchase. But if you really want a slim they are better built than the first 360's if a little plasticy. Just beware of the dumb children on Xbox Live that will burst your eardrums.
I say get a used PS3 or 360, either will probably be a decent purchase.