I play Age of Empires II and III on my Macbook and with an iMac over LAN. I'll tell you I prefer AoE2, but if you're a more casual gamer, you may like AoE3 better (with its MUCH better graphics and more simplified gameplay).
There is a ton of user-created content out there for AoE2, and almost none for AoE3. IMHO, that's the reason, at least on a Mac, AoE2 is a better choice. The Mac version is basically identical to the PC version, except it only doesn't do multiplayer with PC's. So you can put in all the cool custom AI's, mods, random maps, and scenarios created by PC users on your Mac.
My Dad (really into military strategy) prefers to play AoE3 with me, because he's more of a casual gamer. However, I'm always up for a game of AoE2, because between some good custom AI and a good human opponent, you'll have some fun.