Very sweaty isn't really a problem unless there is excessive heat like luvs2ride said. When you take your helmet off, is your head all sweaty? Probably...and it's no big deal!
If you use a lighter (I'm guessing thinner) pad, your horse will still sweat, and it might soak through, staining your saddle.
I've always liked sheepskin (the real stuff!) pads for keeping my horse's back cool and cushioned.
Here's an informative website about the different types of saddle pad materials and their benefits/faults: http://www.5starequineproducts.com/S...dsArticle.html