Unless its changes recently, I was under the impression that the core CoH rules applied to all the games?
I haven't read all the manuals so I wouldn't know honestly. I "assumed" there would be differences due to differing units. But you'd still have to use and disect the manual depending on the game you're playing to find those differences - I'd imagine. The core rules may well be the same though.
In all honesty, I flim flam all the time about which games I want to play. In recent months I've had the manuals out for Heroes of the Falklands, Heroes of Nam, CoH, CoH Solitaire, DVG Warfighter:Modern Warfare (which is still by my bed) and - more recently - LnL Blood and Bridges. Truth is, I want to play all of those, but they're so vastly different in ruleset that I can't make my mind up which one I want to play.
I've currently settled on Blood and Bridges because I bought into the LnL Kickstarter - but god alone knows if I get to play any of that out.
It doesn't help at the minute that I don't have a proper gaming area (as I converted the lions share of my room into a modelling work station).
I just had some birthday cash spare and thought I'd chuck it at a board game that I could look into in the future - otherwise it'll get spent on something less worthwhile