Paul, yes, there is quite a choice of track available in every scale. If you are purchasing it locally, then you will probably want to stick with what the local shop carries the most of. If you are ordering online, then you can choose whatever you want.
For sectional track, Kato (with the roadbed molded onto the track) is quite popular. Its a good, quality alloy, and since it has the roadbed molded into the track, its rather sturdy. But sectional track can be quite limiting. Most times its difficult to mix different brands of sectional track with the roadbed molded onto it, because each brand locks together differently.
For flex track, most folks go with the tried and true Atlas. I used Micro Engineering track on a small set up, and its also a good quality flex track. I like the Micro engineering stuff better then the Atlas, simply because the Micro Engineering stuff has a proper rail profile. As with sectional track, sometimes it can be a challenge to mix brands of track. Because even if the 2 brands are the same code (height of the rail), they can have different dimensions in the rail foot and web. This will make it difficult to properly join them together with rail joiners.