The difference is perhaps best explained in an old review, http://188.8.131.52/search?q=cache:E75pF2AgovMJ:www.gamesfirst.com/reviews/rick/CWG2/CWG2.htm+%22cwg2%22+%22Sid+Meier%27s+Gettysburg%22&hl=en&ct=clnk&cd=2&gl=us&client=firefox-a.
These are truly the best of times for Civil War gamers; first Firaxis comes up big with Sid Meier's Gettysburg!, with its real-time tactical combat engine, and now Sierra delivers the goods with Civil War Generals 2, a turn-based game that allows you to refight the entire Civil War - battle by battle, campaign by campaign. Make no mistake: the two games are very different, both in scale and in gameplay, and in many ways Civil War Generals comes off looking less sophisticated than SMG. The graphics are a bit dated, the game's mechanics are scarcely revolutionary, and the game interface is oh so Windows 3.1 - but the bottom line is that CWG2 does what it sets out to do. That is to produce a fun, historically accurate, good-looking game that allows you to fight every major and most minor battles of the American Civil War at the regiment/brigade level.