Well Empire Total War for one has naval battles, something RTW does not. And the graphics are stunning! Your own guns can blow to pieces and kill your men though.
Apart from that, there are roughly fifty factions I think. So indeed, the game will be epic.
So does Empire Total War give you a much larger variety of factions to choose from? I know that Rome Total War only allows you access to factions that would have historically interacted with the Roman empire.
Apparently yes. You can choose...Sweden, Britain, France, Austria, Russia, Ottoman Empire, Spain, Prussia a variety of small German States and mroe I guess.
These were just a few of the factions that I currently know of.
I haven't played it yet....but you can use different types of shot. Grape-shot, normal cannon-balls and the like.
The most number of ships you can deploy if i don't recall wrongly, is around 20. Which is pretty realistic, as the battles of that time were really small.