For basic use, a simple Express will do, as long as you have decent WiFi. If you must use wired Ethernet, then only the Ultra has that. The Ultra is a bit faster, but all the others are about the same as far as performance. It's just features like included remotes (IR vs WiFiDirect with voice support.)
Also, the main model number is all that matters. Letters at the end, like X, R, RW just tell what market it was for (X=Roku direct, R=Retail, RW=Retail WalMart.) The differences are usually limited to which shortcut keys are on the remote, such as the WalMart having a Vudu button (WalMart own Vudu).
