Also read up on the TW Unit Guide Frogbeastegg wrote up:
https://forums.totalwar.org/vb/showthread.php?t=31444
It has both STW and MTW units in it, it's a great way to get a feel for the units and their uses.

The Viking Invasion campaign is shorter, with fewer provinces to manage, but can be more difficult depending on the faction you choose. It's a blast once you are used to it though. The main MTW campaign can take a while to finish, depending on how much time you have to play.

Welcome to the dark side!