Finally, something I can offer an opinion on without being totally uninformed. I live less than a kilometer away from Harvard Yard, the center of the Harvard undergraduate campus. I applied to both Oxford and Harvard, got deferred at Harvard and accepted to read law at Merton off an open application. GO TO OXFORD. You must be crazy even to have to think about this.
As I'm writing this, it's snowing. I mention this because it is freezing here, always. I can't stand the winters in this godforsaken hole. Oxford may be grey and wet, but rather that than frostbite.
In regards to the social scene, it's not impossible to get alcohol when under 21; just find someone old enough to get it for you. That said, it's awful not being allowed into bars. Cambridge is a decent enough place for students, but downtown Boston is dead as far as cities go. There's little club life, not much interesting stuff to do and mediocre public transportation.
I've taken a couple of night classes at Harvard and was never blown away. Yes, the profs. are good, but all classes for undergraduates are lecture based and have tons of homework. I much prefer the tutorial system, but that's open for debate.
Architecture: Oxford wins, hands down. While I disagree that Harvard's buildings are all "ugly" and "too modern", they don't hold a candle to Oxford's splendor. And of course 17th century buildings, while considered ancient here, are probably "modern" by Oxford standards. Still, that's not a real factor to take into account with such an important decision.
If prestige is important to you (and let's face it, it's important to everyone) then it's a wash. Both schools earn respectful whistles of awe when you say you went there. Americans in general view Oxford as being somehow "better", though; we have an inferiority complex.
I might have to make the same decision as you, if Harvard lets me in on their second round of acceptances. However, I'll have no problem making my decision. Oxford wins hands down. You can't meet Sebastian Flyte at Harvard, now can you? Then again, at Harvard you don't have to make an offer...I'm not looking forward to trying to make mine (3 AP tests at grade 4)!
Also, knowing many Harvard students, they're pretentious buggers. If you want to know anything about Cambridge or Boston, message me.