Napoleon: Total War is easily the most divisive of all Total War games. Some folks love it, some folks hate it. It is, to be fair, an almost carbon copy of Empire at first glance. The most noticeable difference is that Napoleon concentrates exclusively on Europe whereas Empire allows the player to conquer the globe.
Beyond first blush, Napoleon does feature some improvements over Empire that make it worth consideration. Chief of those is better coding - Napoleon is accepted by most to be a much more stable and better running game than Empire. It is also better looking. AI is supposedly markedly better, however that is debatable. There are a number of mods available that greatly improve the Napoleonic experience - but you're fine running the stock game.
If Empire and Napoleon look too similar to you - if muskets and dragoons are all the same to you, you may prefer Empire over Napoleon due to the globe. But... If, like me, you have a fascination with the Napoleonic Wars, you owe it to yourself to give Napoleon: Total War - beyond dominating Europe, there is also a series of historical battles and campaigns that are very enjoyable.