Yep I do those. Either go through with new games on each, on hard if possible, or load up challenge mode for each.
FFA is good fun but the campaign has, and this is going to sound daft, less story to it but it probably takes longer on the first playthrough. If you're a trophy hunter or going for all the skill points then it'll be way longer than QFB. And it has multiplayer! which also has some skill points.
But I have no friends to play with ór a PSN accound
The PSN-thing is my own choice though and I think that €50,- is way to much for a controller I buy for someone else.
As far as I know, you can't even start over. I wanted too because I accidently skipt a cutscene in the beginning, but I couldn't figure out how to do that.
Just make a PSN account, it's 100% free, and there is much fun to be had!
And yea it sucks you can't start over. The easiest thing is to just make another PS3 account and play it on that. That's what I do on A4O if I want that feeling of starting from a clean slate.