I myself have bought the iron cross expansion at a time when DH was not on distribution yet, as I played DDARMA to some other sphere before.
Iron Cross isn't a bad game at all, but in most aspects of gameplay, you'll have a MUCH better experience playing DH without IC. As I read from your avatar, you haven't bought DH yet. In this case, just try DH, it isn't that expensive, and soon you'll see the greatest benefit of DH: The HoI II engine without lag. That's perhaps still at patch 1.03 the greatest thing one could ever do for us: Make HoI 2 a non (at least at most) lagging game!
I have tried [IC over DH] thus, and can report: It's better than [IC over DDARMA]. It has (only personal opionion) some lagging advantages over [IC over DDARMA]. But it hasn't got (as far as I noticed) the substantial content of the decision making interface, and that's a thing I don't want to miss anymore, just after playing the first match of DH.
Conclusion: Both games have their "right to exist", but in my opinion, DH without "addon" is the better alternative. If you want the "500000000000" flavour events of IC though, make a copy of DH and install it over it, as the IC installer is ready to copy over an installation, in spite of having another version installed. The IC installer is in complete very reliable and easy to use, making it easy to install and test both versions (many thanks for that)