I personally like fleece girths over leather ones. I think they breath much better then leather and they even come in CoolMax these days. Honestly though, when it's hot it's going to be hot regardless of what it's made of.
I'll probably get flamed for saying this but I don't think most fleece pads add so much bulk that it completely alters the fit of your saddle. But then again my saddles have to fit with a fleece pad (for shows) and I school every horse in a half pad/square pad so maybe my saddle is a touch wide.