.204 is fine, as is many calibers. Now, AR and thermal is not synominous with light weight. Thermals are heavy optics in comparison. The AR can be/is heavy in comparison to most bolt guns. Also, the mag, and pistol grip on an AR can make them awkward to shoulder for long walks. I hunt with both and love the AR platform. Just be prepared to be heavier than most calling rifles.
As far as brands, I am a Rock River fan for production AR rifles. There are many other brands out there that are great also. Having said that, most, if not all, production .204's that I have seen are going to be a heavy varmint barrel, adding even more to the weight.
Others will chime in, but this has been my experience.