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!
I own a number of cars from PNG as well as about 45 AMS cars. In general, a well built PNG car may be somewhat more durable than AMS but it is pretty close. Both cars are very well detailed.
For me, if the car I want is available from AMS I buy, but if I need something special kitbashing or whatever, the PNG cars make a wonderful starting point. The extra few dollars that a PNG kit costs does not detract at all from what is very good value for the money.
I agree with Josef and Doug. I have only one PNG car, a D&RGW 40' reefer. I built it last winter and spring. I really like the looks of the car. There are a lot of parts that come with the kit. The wood is all precut. It took a while to put it together. There is a lot of satisfaction in running a car that I built. I have a number of Accucraft cars, both freight and passenger. The level of detail is comparable. It you like building kits go for PNG, if you like your cars to run out of the box go for AMS. You can't loose with either option.
If you are interested in a wood verses plastic comparison, I'm doing a thread over on the Finescale Railroader forum [http://www.finescalerr.com/] where I'm detailing and weathering an AMS high side gondola, then building a board for board wood and metal version of it. It is called "A tale of two gondolas".