Good question. You wait for ages for a good Napoleonic game then two come along at once!
I have been fascinated by the napoleonic period for many years and enjoyed 'Imperial Glory' when it came out back in 2005. It was the first strategy computer game i had played and i was impressed with the eye candy, the breadth of the campaigns and the naval battles, despite all the obvious draw-backs in the land battle realism -lack of units routing or rallying but fighting to the last man etc etc...Then i discovered the total war series which had better battlefield behaviours, but not as good graphics at that time, and I hoped total war would focus on the period, but contented myself with the Lordz napoleonic mods of the existing total war games (Napoleonic total war 1: a mod of Medieval total war 1, and Napoleonic Total War 2 a mod for Rome total war). Both had limitations due to the nature of the underlaying game mechanics set in the age before gunpowder warfare, but they were enjoyable games and NTW2 was visually impressive- a real labour of love by the modders. When Empire TW was announced I thought that it would cover the Napoleonic period with the sort of impressive display that I had found with Medieval total war 2 and was excited by the prospect of the combination of high end graphics, the inclusion of naval warfare and the campaign aspect but after a long period of anticipation when the launch came i discovered that while my system could probably run it, the whole DRM system using Steam which they introduced just for this one, effectively excluded me, that and the hugly negative reports from users post launch ( need to download 100Mb patches at the machine on which you play , need to update online, often when you dont want too etc) became a deal breaker for me...I suspect Napoleon Total War will also have the same DRM, and huge graphic and memory requirements which would mean I would need to buy a new system to run it well and would need my gaming machine to be online even to play in solo mode...I regret this as I really enjoyed medieval2 and hoped the Napoleon would be the same sort of thing but in my favorite period.
I know Empire has subsequently improved now it has been fixed and judging from a few of the hugh number of youtube in game clips it can be visually impressive, looking film like in some battle sequences. But the combat is still limited to the old micromanaging 20 units so it may reproduce well a division size battle but not the sort of corps level actions we can play with HWLG.
The demo of HWLG runs like a dream on my system (under windows XP) and doesnt require online DRM of the sort that put me off ETW. We can play proper Napoleonic size battles in a way that it will not be possible with NTW, the unique fog of war aspect in HWLG really turns it into a simulation of napoleonic warfare...The graphics are not at the photorealistic level of NTW so they do not require the computing power that program will need to run well. They are good though, and in fact look as good as TW graphics at the usual distance the camera holds as we fight a battle...
Needless to say I have already purchased HWLG on the basis of the demo and what it promises when all the minor bugs are ironed out. I am already enjoying playing the Demo 2 ( pre-released to those who preorder) which has fixed some of the Demo 1 issues.
HWLG and NTW running at their design best will be very different sorts of game, the one emphasising realistic simulation of large scale battles the other a large scale campaign dominated by small scale battles....You pays your money and takes your choice!