One of my grandmothers made quilts with poly blend, and they are durable and bright and seem like they'll last longer than cockroaches. BUT they don't get that super soft touch to them like aged cotton does, and the lack of fading is kind of a drawback to me. It's just not quite as "cozy" compared to her cotton quilts that are of the same age and that show their age much more. So for me, I prefer cotton because I LIKE that it gets old and soft and faded and worn. But it's just a preference, I don't think either is "wrong".
I haven't ever noticed that her poly/cotton quilts were warmer than the all cotton ones. But I think she used poly batting in all of her quilts and I imagine that'd hide any difference coming from the pieced fabric's materials. (Or maybe I'm just not very observant, lol)