I've got both, but I don't really play either anymore.
Honestly. I'd suggest you don't buy either...
Warhammer: Age of Reckoning: I'd say don't buy this game, I played it for about two weeks before I got bored. The whole game is basically BG's and Public Quests. As it stands the games lost like 50-70% Of their Subscribers, I know it was atleast like 200k.
Age of Empires 3: I like the game, but if you plan to play online I wouldn't suggest it. The Onlines extremely buggy, and not many people play it anymore.
Alternatives IMO
For an MMO I'd suggest waiting for
Starswars: The Old Republic, Global Agenda, Final Fantasy XIV was just annouced too. I haven't played this games, but the look pretty amazing, and after the recent E3 theirs been a lot of good hype about Global Agenda(Mainy GA, because the others don't come out for a year or two).
RTS Wise, I'd strongly suggest waiting for
Starcraft 2 to come out, guaranteed to be the best RTS of the year

.