Stainless all the way. I don't carry much in the way of gear when I'm out in the bush so the gear I do carry needs to be indestructable. I carry a stainless canteen with a stainless cap and a fitted GI cup on the bottom that is also stainless. I can boil water, make stew, dig with the cup, and one time I got a nasty cut from a saw palm and used my bandana, which also has multiple uses, as a dressing. I built a makeshift pressure cooker by making a dakota fire hole and then digging another hole really close to the fire and buried my canteen in it with only the cap sticking out of the ground. I then grabbed a 10-15 pound log and laid it over the cap which was turned upside down so it would lay flat and seal it. After cleaning the bandana by boiling, I folded and tied the bandana so that it remained about 1" above the water line to receive pressurized steam and sterilize it. I let it stay in there until I boiled about 2" of water out of the canteen (maybe 30 mins) and I am pretty confident that my bushcraft 'autoclave' worked well since I never got an infection and it healed up really well. Using that method I was able to sterilize my bandana twice a day for 3 days. I wanted to share this just because it's a good tip to know if you were ever in a position that required sterile technique in a wilderness environment. The only problem I had was, despite the weight of the log, when steam released it sometimes tried to shift the cap and I had to fiddle with it to make sure it kept a decent seal. Since then, I carry a peice of bendable pipe strap that you can get from the hardware store so if I had to make this again, I could wrap the piece of pipe strap around where the cap and canteen mouth meet so the cap could still move vertically to release steam but not shift to the side and create a hole where pressure could be lost. If someone were to think about it, I'm sure there's a better way to use a canteen to make an auto-clave and IMO a worthy project.
I've used my stainless cup for making lamps from animal fat and I've even used that cup to boil a bunch of wax myrtle berries, and after separating the water, used the cup for a wax candle.
One other unique use that I can think of that probably would not work with Nalgene is that I use the cup for a perimiter alarm to alert me if a curious bear is coming into camp. Basically it's a dog-bone trigger trap that releases wound tension in paracord. When the cup is tied to the paracord and the trap is tripped it causes the cap to spin around and strike a well placed object making a hell of a lot of noise. Every time I go camping anywhere that is not a campground, I set one of those perimiter alarms.
Nalgene might be pretty, but my stainless canteen will eventually become a family heirloom. It'll be around long after I'm gone.