I think Microsoft has extended the warranties for 360s after the whole RROD issue.
If you hardly use your PS3 now, I don't know if buying a 360 is going to revitalise your interest in console gaming, After the novelty wears off, you might find yourself with another expensive paper weight.
PS3 exclusives generally look better then the 360s, but that isn't to say the 360s are chopped liver. Exclusives sell consoles to certain demographics or fanboys. Unless there is some particular 360 exclusive you just need to play, I don't see the need to get one.
Multiplatform games do usually look better on the 360 because it is usually chosen as the lead platform. Some games are slightly better, some are overwhelmingly better.
Fan noise seems louder on the 360 than the PS3 for me. I have the newest 360, around 6 months old, and a slim PS3 I've owned for about 3 years. Sometimes I can hear the 360 rattling away after a long session. I've tried both consoles in the vertical and horizontal position; it's true either way.
Don't mess with modded consoles. It might seem tempting, but I advise against it. Running homebrew and emulators gets boring after the novelty wears off. As far as backups are concerned, I think that is a silly justification for modded consoles. I have dozens of games in mint condition because if they aren't in the console, they are in the cover. Maybe you can run games off the HDD without changing discs, but I don't think that small inconvenience is worth the risk of getting a banned machine.
From your post, this seems like a passing fancy. If you got plenty of cash to throw around and you rarely suffer from buyers remorse, have at it. Otherwise, stick with your PS3 and look into getting some exclusives/MP games you mightn't have played yet.
Hope that helps