Its really a personal / monetary preference. if you go with O standard its a meatier track with several switch options. 0-27 is a little more light weight but your switch options are a little limited. if you go with Ross Custom, well the sky is the limit, but it more pricey. Fast track is easy to put together, but it takes up more room then the other tracks. You usually can't mix track types, Ross has adapters for O & O-27.
I am by no means the authority on track... this is just what I have found in my travels... I personally use O-27 since its readily available, and you can pick up cheep batches of them on E-bay.