The thing that really bothers me and holds me back from getting a lot of stuff is that MS points are not sold in 400 / 800 amounts - an obvious marketing ploy.
Right now I've got just under 800 points but can't be bothered going out to the shops to get another stack of points just for the few I need.
Personally I'd go after Mass effect 2 stuff or RDR. But get what you think you'll get value out of.