I will say first off, I'm probably a bit biased towards the 360. That said, onwards.
It really depends, I bought my 360 last summer when Assassin's Creed came out and was blown away by it. Picked up Gears of War
, Assassin's Creed
, Mass Effect
, Dead Rising
, March Madness
, NBA Live
, Halo 3
, Rock Band
, and Lost Odyssey
all within about a four month period (hey, I get $1700 a semester back from grants!). All of them I've sunk countless hours into (sadly). I later picked up GTA IV
, and with it I bought a year subscription to XBL ($50).
I bought a PS3 back in December because I needed a blu-ray player. It was a phenomenal blu-ray player and what movies I have look great when put on a big screen TV (and I mean
great). I played Uncharted
, and it was fun, and Metal Gear
obviously rocked. I recently sold it to my friend simply because the exclusive lineups weren't that interesting to me, and most of the shared games I ended up playing on my 360. If I had bought a PS3 first and hadn't already owned so many 360 games, I might've been more inclined to keep it.
Having had both systems, games played on the consoles honestly look about the same. The PS3 has higher specs, and should look better, but in actuality games look about as good on a 360, a few look better, a few look worse. I primarily play sports games with friends and RPGs by myself, but Halo 3 and Gears of War really stuck out to be as being fun, solo and with a few buddies, and with FF XII coming to the 360 I had no compelling reason to keep my PS3. I'm similar to you in that I only play online infrequently (GTA IV online is a lot of fun though), so the PS3 wins in that regard, but I had $50 to dump and figured what the hell, it's only like $4.11 a month, and the online play has been rock solid. I can't comment on the PS3, but I've read it's getting major improvements in the next few months.
In summary, personally I find the 360s exclusive games more appealing than the PS3's (God of War will be cool too), but there has been rampant hardware failure in the 360. Apparently the newer models have been fixed, as mine is ticking just fine (it is fairly loud though, the girlfriend complains but shoot, I gotta have my games
). I miss the blu-ray capabilities, but I couldn't justify a $500 blu-ray player just yet.
For sports games, either should do fine. It mostly comes down to exclusive titles and if you're interested in a blu-ray player. One thing to note, on either system, is to make sure you don't cram it in a closed entertainment center. Both consoles need to breathe, but the 360 especially needs a fair amount of air in front and behind it (so no shoving it against a wall).
I also own a Wii, but someone above described it perfectly: after the novelty of using your hands wears off, you quickly realize why game controllers have excelled for so long. My girlfriend absolutely loves Boom Blox
, so if you have someone that isn't into hardcore gaming at your side, they'll probably enjoy the Wii more than anything the 360 or PS3 has to offer. Great fun at parties, though!