Are you guys kidding? Shogun 2 is the best TW to date in terms of graphics and polish, building on the past TWs. It had both the gun era and old sword era how is that for unit diversity? Rome is a classic but outdated. It's up to Rome 2 to take shogun 2's crown which seems to be the case so far 
I payed 500 sek to play a game with 1 unit roster. Oh and after that they added a new unit roster by DLC with the Ikku Ikki, and then they added the rest through expansion and another piece of DLC (rise of the samurai).
Rome have 7 rosters with different units (Chartaginian, Barbarian, Roman, Greek, "Eastern", Selecuid/Macedonian and Egyptian) and old fans will probably call that generalising the factions a lot, since every faction got largely different units (except for the 4 roman factions, they got 2 different rosters though). All the barbarian factions got 2-3 units different from each other, the Selecuids are very seperated from Macedonian etc.
Shogun was a good game and I enjoyed it. It was way too streamlined though (couldn't move freely through territories made the map feel like a series of narrow corridors) and after one playthrough I got bored with it. In Rome I felt I could (and still can!) play it 100 different ways depending on how I want to deal with Rome, were I want to expand, what I want to focus on etc. Shogun didn't have that and felt more like a expansion than a full game. (See: Total war: Alexander/Medieval 2: Kingdoms)