Check out luzme.com to get a comparison between the different ebook stores.
I usually add books to my Amazon wishlist, which is connected to my luzme account, so I get notifications whenever the prices go up or down and in the individual stores.
Whenever I can't wait and I need a book right away, but the paperbook is around the same price or lower, I'll buy the paper book and donate it to my local library after reading it. I much prefer ebooks, but at least I know someone else will profit from my purchase as well afterwards. If they are books I think I may want to reread, I buy the ebook version, though.