First off, I own AoE2 and AoE3 and play both often, both in single-player and over LAN.
AoE3 is all about graphics, but lower on features. Terrible AI. User-created content is difficult to make, and therefore there is not much available to download, and strategy is not as important as simply cranking a build order faster than your opponent. I still enjoy playing AoE3. But every time I do, I find myself thinking, "if only AoE3 had *this* AoE2 feature."
AoE2 has pretty bad graphics, but they are acceptable and do what they were meant for perfectly: telling units apart and getting an idea of the setting of the time. And is an RTS really about graphics anyway? The gameplay is much more centered on strategy. There are diplomacy options (AoE3 had absolutely zero diplomacy options), tons of maps, scenarios, mods, ect. to dwnload (that will work on a mac, unlike AoE3). The AI is still bad, but you can download custom AI (unlike AoE3, where custom AI isn't supported), and I play with one that can still beat me a year later.
AoE2 is definitely harder to play. I always teach friends AoE3, for that reason. But AoE2 keeps you coming back for more, and unless you're playing seriously online, AoE3 eventually won't do that for you.