Greg,
I do not own any products from Phil's Narrow gauge, however, many of the members of a club I belong to have PNG rolling stock. By all accounts, they are far more durable than anything that hartford products offers. Hartford kits have a reputation for being very fragile. As for AMS, they are hard to beat due to the fact that the cars come painted and lettered and are cheaper than the kits (most of the time) and are pretty durable. Another added plus is that AMS letters cars for different numbers, so you could have a whole train of D&RGW stock cars and each car would have a different number. Detail levels between the two companies is pretty much the same, there is not much on the prototype that isn't included on the model. However, building a kit does come with the satisfaction that you built it yourself. I hope this is helpful. Good Luck!