Ereaders that support DRM only support one flavor of DRM. Unless you get a tablet, and install multiple apps from multiple retailers (like the Kindle app and an ePub app).
Most books (except for a few independently published books) are available in both ePub and Kindle format, so where you buy books isn't much of an issue. Also, the major publishers set the prices, so it doesn't matter where you buy the book, you will pay the same price.
Personally, for the buying environment, customer support, and ease of use, I prefer Amazon/Kindle, but others prefer the various ePub readers (Sony, Nook, etc.).
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