Re:Ambus! or Advance Squad Leader?
Having never played Ambush! but owning ASL I'd say the answer lies in what you'd like out of the game.
Ambush! strength is that it is probably the best solitare WWII squad-level game. ASL's claim to fame is being one of the most detailed multi-player WWII squad-level game covering all theaters of operation.
Ambush! would probably hit a cord with you right off the bat since you only need to convence yourself that you should play the game. ASL would be a challenge since it's a complex system that would be difficult to learn on your own without a vet around to show you the ropes, and then trying to teach it to someone would be a task too. ASL did have a solitare module but it was not as good overall as how Ambush! works.
Hope that helps out a bit.