I've managed to wangle a business trip to Singapore over the grand prix weekend!
Like you, Jovanotti, I'm trying to work out which tickets to go for. I did General Admission back in 2010 at Silverstone and have to admit I didn't enjoy it too much because I was standing the whole time. I'm very fidgety so I hate standing still, and when it comes to race day if you don't have the willpower to stand still for hours I doubt you'll get a decent view.
If you haven't read it here is last year's GP guide from Grand Prix Advisor, it has some useful information on the grandstand options: Singapore circuit guide
I like the look of Bay Grandstand but it seems so much cheaper than the other grandstands I'm wondering what the catch is? Personally I think all the other grandstands are too expensive for what it looks like you get, it's not as if Singapore has an iconic corner like Eau Rouge. I do like having a seat so I'll probably go for Bay, if not I guess GA Zone 4. Apparently that zone is near the main stage (which I was told today is near the Padang grandstand) so it's good from an entertainment point of view too.
Also, Justin Bieber can happily be ignored. He's performing on Monday evening and you need to buy tickets separately to get in, so I don't really see how it's connected to the F1. Performers during the actual GP weekend include The Killers on Saturday and Rihanna on Sunday.