Re: Empire Total War vs Napoleon Total War
I agree with Daruwind that Napoleon Total War campaigns tend to feel the same. To be fair to the developers, I'm not surprised by that, as of course NTW is meant to simulate a specific historical war, compared to ETW which simulates a century of warfare (or more, if you include part 4 of the tutorial campaign which starts in the 1780s and continues into the 19th century).
As Daruwind said, there's much more of a sense of technology moving on in Empire. This makes sense, as the two games cover very different amount of time. I agree that the AI doesn't build forts in Napoleon. While I know that siege warfare was historically important in this era, I see this as a good thing. As I see it, the siege battles in Empire tend to reward the same set of tactics, so they can get repetitive.
You asked about variety. As I see it, there's more variety in Empire. Fighting Native Americans feels quite different from fighting in Europe, which is different from fighting in India or north Africa. Native American factions tend to rely on melee infantry and stealth. European factions tend to rely on the traditional mix of line infantry, cannon and melee cavalry (some have ranged cavalry too, some have more/earlier access to light infantry, some have better line infantry especially later on). Factions in India tend to use a mix of melee infantry, ranged infantry, cannon and cavalry. Factions in north Africa tend to be easy to defeat (they have poor quality infantry) but their regions tend to have frequent rebellions, so they can be easy to take but hard to hold.
I think CommodusIV summarises the strengths and shortcomings of Napoleon well. I do enjoy the main campaign in Napoleon and I think the battlefields can be more interesting in NTW. Multi-player may be better in Napoleon (I really like the ability to play 'drop-in' battles in Napoleon, where you can play the enemy in someone else's campaign or vice versa, although I've rarely been able to do this.) I enjoy the Napoleon tutorial campaigns too (in Italy and Egypt) and the historical battles, as well.