Which game you choose depends mostly on which period of warfare you prefer. At least one person who loved the American Civil War has told me that after playing Napoleonics he couldn't go back to ACW. I much prefer Napoleonics myself because the warfare of the period had a much greater depth than ACW. There is a lot more to do. As an example, just having cavalry that force infantry to form square makes a major difference.
The Waterloo engine shows a bit of polish and improvement over the ACW engine, but someone else would have to tell you of any major improvements. If you get Waterloo, be sure to get the latest version of the Grog Expanded Toolbar V available on this forum. It makes a lot of improvements to the original game, and an earlier version of the toolbar is included with the game so it is considered official.
The AI will never do exactly what you want, but what it does do is good. I've fought the full battle of Waterloo many times (I'm on #20 now), always under as much AI control as possible, and while people complain about the AI it has impressed the heck out of me. The main trick is not to interfere with it any more than you have to. Of course once you master the game you can beat the AI every time, but that takes a while.
For what it's worth, I've never lost the battle playing as the French except twice when I intentionally avoided taking any VP sites. In both of those games I gave an attack order through Napoleon to everyone except the forces sent to contain the Prussians; I do not recommend this course of action. In the second battle I only gave one additional corps commander an order.
The game scale isn't exact. Because of the 200 sprite per battalion limit you aren't going to get every man on the field. Anything over 4:1 makes the infantry formations a bit too big, while the cavalry formations will be a bit small. At 4:1 an infantry line is the correct scale for the map. I've tried 1:1 and the infantry lines are just too big and unwieldy even with a maximum of 200 men. I've tried other scales and currently prefer 3:1. Don't worry, there are certainly enough troops on the battlefield and the various formations are about the correct size for the map scale.
It was easy to get into the game. I played through the tutorial twice, then a couple of brigade level sandbox games, then a corps level sandbox, then jumped right into the full battle and even won. But expect to be reading the manual and trying things out for a long time before understanding everything.
Hook