Col Bier,
The HPS games graphics are not as good as the BG ones since the maps are drawn using a tile system instead of being bit maps. Personally being an old boardgamer I almost always played in 2D mode anyway so that doesn't bother me much.
The game engine is a bit different offering a single phase system where you can move, fire and melee at any time during your turn although once a unit fires or melees it can no longer move. The system has an opportunity fire for the non phasing player's units to shoot during the other players turn. This is the weak link in the single phase system as units often do not fire as enemy units march forward. Used to be your units would not even fire before melee although this has been fixed with a recent patch that has full fire before melees and the opportunity fire does seem to have improved a bit with the recent patches. This has made the games a bit better in this mode although there are still times I get upset about my units not firing. The single phase ssytem does allow you to change formation at anytime during movement and cavalry can dismount and fire on the same turn. The other problem with the single phase turn is it is subject to gamey tactics. An opponent can use units in his second line to move forward, melee your units to blow a hole in your line then exploit the hole to strike your second line or surround your front line units. Blitzkrieg in the 1860's.
The game does have the option to play in phases just like the BG system although you lose the ability to change formation at any time and the opportunity fire. It does have some good points, Fixed units release if the enemy is visible within a certain range ( 5 hexes I believe) so you don't have to wait till your opponent fires at you to egt them released. The campaign system although not perfect does force you to think about more than the current battle since casualties are carried through to the next fight.
The one thing that keeps me playing them is the respose I've seen from John Tiller and company. I was at Tiller Con in Virginia last summer and sat in on the discussions on the ACW games. Several of the things brought up there have already been included in the latest patches as optional rules. Things such as the Final Fire before Melee mentioned above, a density fire modifier, limits to what can cross bridges and the ability to repair damaged bridges have been added to the games recently.
I have problems with some of the things in the HPS game engine and I still play BG games occasionally. I also have another real time game that I think is a better ACW simulation but unforunately it's only a solitare game and I prefer to play against another person. However until something else better comes along I will be playing the HPS games too. Hope this gives you an idea of some of the differences between the systems. I've probably left out a lot and I'm sure others will give you more to think about.
Gen. Ken Miller
Army of the Shenandoah