Have a bunch of American Models & SHS track....which one would be better to use on a new layout?
Well, that's going to depend on how much your "bunch" is or what size of layout you have to work with.
You don't mention if your going to use scale-wheeled equipment or HiRail/Flyer equipment. Track availability from all manufactures can sometimes be spotty if you need to add-on.
American Models track is good stuff, but you'll pretty much be married to it. It's rail height (code 148) is higher than SHS(MTH) ScaleTrax, Lionel's S Fastrack or Fox Valley's track (all code 138.)
SHS ScaleTrax is now manufactured by MTH. Also good track. Turnouts are only available in R20 and no crossings are available. The good thing is it is compatible (with a little work) to Lionel's S Fastrack.
Lionel's S Fastrack has slightly more variety in that 45 and 90 degree crossings are available, plus turnouts in R20 and R27.
Fox Valley Models is a relative newcomer to the track scene and manufactures flex track and No 5 turnouts that have the same rail height as S-Trax or S Fastrack.
Unfortunately, there's no easy answer. All of the track systems have their advantages and disadvantages.
I would suggest American Models track as they offer a much larger assortment of track options.
Also made in the USA? I cant remember for sure