Bottom line is, regardless of which you buy, as long as you buy 1 of the 3 CoH's you can play multiplayer with anyone else, regardless of their combination of owned CoH's.
For example if your friend has opposing fronts only and you have tales of valor, you will each be able to play together, but will have different units unlcoked in multiplayer as a result.
Heres a breakdown for you.
CoH Original (Vanilla) = US and Wehrmacht teams in multiplayer
CoH OF = British and Panzer Elite teams in multiplayer
CoH ToV = Several optional custom units for each of the four teams listed above.
If you still have questions let us know.
-Blue