Both are good track systems which can produce beautiful results. Look online and on YouTube for examples.
I have used Atlas-O, and have some Scaletrax not used.
I agree that historically, Atlas-O has had better availability, but that that is not a prediction about future availability. Check some online vendors for their stock and price just to see.
Scaletrax has thinner better looking rails (and thinner center rail) than Atlas-O. Atlas-O has accurate tie spacing and size. To my eye, the better tie spacing of Atlas-O is my preference, but is "in the eye of the beholder". Atlas-O has improved their rail-joiners with a small indent to increase friction and hold against the rails. Scaletrax uses small metal springy contacts to carry power between track sections. Both have flextrack which can be used for curve-fitting and easements. Atlas-O has a much wider variety of curves and partial curves, and a wider variety of turnouts: this makes track planning easier with Atlas-O.
-Ken
Wishful thinking: Perhaps my ideal would combine features of both: use the rail of Scaletrax with the tie-spacing of Atlas-O with rail-joiners to join sections of track. With blackened center-rail. With the wide variety of Atlas-O sections and turnouts. Most of my actual layouts and layout plans have used Fastrack because of the wide variety of curves, decent selection of turnouts, and the built-in roadbed - which saves me time and hassles - though on my current layout build, I am ballasting the roadbed sides of my Fastrack for better appearance. More daydreaming: I love Kato's HO and N Unitrack, with their clever uni-joiners which both keep sections together tightly and make electrical connections reliable, and better-looking integrated roadbed. If Kato made 3-rail O scale track with a good variety of turnouts and curves, I would switch over to Kato-O in a heartbeat.
