I like both for different reasons.
PBEM because the Commander is human and as such even if they have a doctrine or two there will be surprises, plus it is easier to come up with different reason d' guerre to play under.
AI games because I can play more turns and battles especially when I have some spare time. Plus I can trial all sorts of ideas and what ifs.!!!
Heck it's all simply grand.
BTW m113apc, if you want to PBEM and you can't keep a schedule so to speak, as long as I know that going in it is not a problem.
I have 3 PBEM Campaigns, one PBEM Battle and a AI Campaign of some sort or other going at any given time so one more irregular PBEM will not hurt.
Bob out