There is no point in getting any total war games aside from Rome / M2TW. I was an ardent admirer of CA until Empire happened.
Shogun Total War - revolutionary new kind of game
Medieval Total War - mostly same game, but redone in Medieval times, whoo!
Rome Total War - a great evolution of an already great game, in Roman times, whoo^2!
Medieval 2 Total War - improved Rome, in Medieval times? With modding made so easy you can basically edit text files and get new results? holy fuck! How awesome is that!
Empire Total War - why does it run so slow. why are troops so retarded. why is the AI so retarded. Maybe mods will fix... oh noes basically no modding support because fuck you modders
Napoleon Total War - fixed version of Empire in a new setting... that should've been part of the Empire? no thanks
Shogun 2 Total War - I heard good things about this, but Japan isn't my thing.
Rome 2 Total War - yay cool graphics?
Medieval 2 Total War runs great, works great, and has tons of mods:
Call of Warhammer (Warhammer Fantasy)
Stainless Steel (proper Medieval setting)
Third Age (lord of the rings
Hyrule Total War (fucking zelda total war)
Westeros (Game of thrones)
etcetcetcetc.
There is simply no point in buying their new games. They've realised that medieval 2 was too mod-friendly for them to make money, so instead of going the support modders way, they want the EA way.
Can't wait till M2TW:K engine is public property ;>