Leather all the way for me. I find it more comfortable, luxurious, smells better and easier to clean.

As for concerns about heat: that's what V-Kool/3M/Llumar/etc tinting + sunshade (if necessary) + air-cond is for.

I have parked my car (V-Kool tint) under the hot mid-day sun for hours without a sunshade, and while it was hot, with the aircond, it cooled down quite fast. And it wasn't as hot as one of my previous rides which was very very hot (no tinting, sunshades fixed, fabric seats).