I prefer MOH, but if you are a big WWII game fan used to pc games, you may find some significant shortfalls in each. Use the search bar, I'm pretty sure there are a few very helpful threads on this topic.
MOH was a great game but short. Better than COD3 imo.
I played and completed CoD 3 and I have to say I was fairly dissapointed. The game itself wasn't up to much. Poor graphics, kinda "been there, done that" that levels. However, after a bit of practice, it does showcase some really awesome FPS controls for the Wii. They really do work brilliantly imo and kinda saved the game for me.
MoH looks far better; the graphics seem decent. Or at least far better than CoD3. I've not played it but I have heard its controls aren't quite as good. It's one I'm planning on renting though.
COD 3 was ****. Don't waste your money on it. I had two copies of this game and both had major glitches in. When the battle gets intense it suffers from lag. Avoid.