Yes, if you have the Steam edition then you have to get the Steam edition of all expansions that you wish to get, now and in the future.

TC, if you like the game, then get the expansion. My advice to people is, if they like the game, get the expansion when they hit 48 or 49, in order to avoid losing any of that level 50 quest/MSQ exp. The only reason to get it earlier would be if they want to make an Au Ra character.