I had black leather in my RX 330. Before I got it, I thought that it would be hotter than beige or gray. Once I had it, I didn't really notice a big difference. (Prior RX 300 was beige.) If you want the black for looks, then go with it. Granted, Chicago summers aren't as hot for as long as in southern states.
I don't think there's a "Lexus leather" versus a "Toyota leather". I've seen two kinds of leather on Toyota products - a solid leather like the 4Runner and the RX, and a perforated leather (little holes all over it), like the Highlander and the Lexus GS. You might have to have the perforated leather to get cooled seats (an option on the GS). I think the leather in the 4Runner is identical to the RX - I don't see any difference in thickness or overall quality.