RE: Coated nylon vs Gore-tex: I have both, and still wear both. My observation has been that the garments made with the Gore-tex membrane tend to be more expensive, but they also tend to be better constructed, and have more flexibility (pockets, vents, zip configurations, etc.). I think it's the flexibility of the shells that I use that's more important than the type of waterproofing.
I have a great shell with a Gore-tex membrane knockoff that I bought at West Marine for boating, but, I find it's nice for hiking too, because I can open various vents depending on conditions. I can unsnap the hood, or roll it into the collar, I can unzip the front, sizes, and underarms. It also has a long vent that runs across the back, with a big overlap so rain doesn't blow up under it. Executive summary, you don't really need the Gore-tex, but, some of the best constructed rain gear will use it, so you may buy the garment because of the features rather than because of the Gore-tex.

RE: Poncho's. As was mentioned, you have to work pretty hard to overheat under a poncho, it's about as much ventilation as you can have (well, maybe an umbrella?). I still have a big poncho in the top pocket of my pack. It can go over me and my pack, quickly. I still tend to wear a hat, rather than the hood of the poncho, except in extreme conditions. The hat needs to have a wide enough brim that the water runs off outside the perimeter of the hole your head goes through.

Another feature of the venerable poncho, you can sleep under one, it's not great, but, it works.