I can talk about my experience with MvC3 and UMvC3. Started out on the PS3 and moved to 360 for Ultimate. First thing I noticed was much quicker it was to find a match on the 360. I noticed less lag as well, though it was never a game breaker on the PS3.
As for quality of people, there are douche bags on XBL and PSN. You play a bunch of people and find the good ones, build a friend list, and enjoy the game.
I played SC4 on the PS3 and I fucking hated how I couldn't turn off voice chat. Voice came through the TV and there wasn't a damn thing you could do about it (even during a match), short of not playing people with mics/headsets. XBL has always given you full control over that stuff. I was able to get plenty of enjoyable matches but I do remember quite a bit of lag on the PS3.
Now I have SC5 on the 360. Had a blast with a 3-person lobby earlier, no lag. Watched a stream with a 6-person lobby going on XBL and the first morning had no lag. This morning's seemed a big laggy, mostly during the loading and/or character select screen, a few times during a match. Was a bit annoying but then again the guy was running a stream with plenty of viewers, so who knows what it would've been under normal conditions.
As for the fee for XBL, there are deals that show up every once-in-a-while on Amazon for $40 for a year's worth of XBL Gold. Plus the 360 comes with either 1 or 3 months free. I know, still sucks to have to pay to play online but personally, I'm over it.
Bottom line, I think no matter what you choose you can have a good time. You might have to work on some stuff depending on which console you go for but in the end it doesn't matter too much.