I personally haven't experienced any stopping or starting, if you do get stopping and starting it's cause of your wifi connection not PS3 online.
Do you have a HDTV? I use my PS3 for much more than games, I use it for putting my movies on, music, Blu ray/dvd player and a web browser (for megavideo or youtube) if you have a HDTV it just makes it sweeter.
I have a PS3 slim after my old PS3 died on me, I have an original Xbox 360 also.
Honestly, There isn't anything other than crossgame chat that Xbox Live has over PS3 maybe a more friendly dashboard but I prefer the PS3 XMB. Put it this way, if my Xbox broke tommorow I probably wouldn't care to much. I wouldn't go out of my way to buy a slim.
It's up to you really mate but Honestly I think the PS3 is the far better console