I have the whole set. I don't know why they would be different prices though, unless one set comes with more books. And yes, you can mix and match the blocks to make all sorts of blocks. There is a Pinterest site with Angle Play quilts. Lots of pretty ones.
http://www.pinterest.com/offthewallquilt/angleplay/
I have EQ5 (the old one) and I have designed a few Angle Play quilts on it. Now.. if I could just find time to make them! LOL.
I bought my sets one at a time, as I couldn't afford the big set at one time, but, it sure saves money to get the whole thing.