I upload all my ebooks to Google Drive, and only have a few in my Google Play Books library at any one time.
Play Books lets you read online from a browser on any device without downloading the book, and saves place between devices, so it is far and away the best choice for me. You can also add epubs straight from your Drive, so you don't have to download then upload something to add it to your library. I like its read aloud function as well. It's organizational abilities are really lacking, though, so I don't keep everything I own in there, just what I'm currently reading or may read in the near future.