I've used the following track systems:
O-27
O-31
FasTrack
Atlas 21st Century NS
IMHO:
O-27/O-31 - Cheepest on the market, easiest to cut custom lengths, fudge-factor. It is not 'realistic' enough for hi-rail layouts. Switch machine profiles are too high. Switches are reliable. Connecting and seperating track over time causes loose connections.
FasTrack - Pricey, but VERY reliable and more realistic than O-27/O-31. Track sections can be connected and seperated ALOT and they don't wear out. Custom lengths can be difficult to make. Switches are DEAD-ON EXCELLENT.
Atlas - Most realistic on the market. Best track to use outdoors as it is Nickle Silver and will not rust. Ties are UV protected. Switches are vast and varied, however I do NOT like their switch controllers. When you are spending this kind of money on a switch you should not get the cheep $.02 HO controllers they include.