First of all, let me tell you what I mean when I say "harem" and "reverse harem" because different people seem to have varying definitions of these two terms which are frequently used in the world of anime and manga.
Reverse harem - is the reverse of a harem in the sense of there being one female protagonist surrounded by three of more men who at one point or other start to develop feelings of love for her. And just like in a harem, all that's needed is a hint of romance or sexual tension between the guys and the girl. Animes I've seen that I would classify as reverse harem are Ouran High School Host Club, The Story of Saiunkoku, and Fushigi Yuugi.
Personally, I like reverse harems better than harems mainly because I like my anime and manga with lots of hot guys in it. And I find the notion of three or more guys fighting over the same girl quite exciting and romantic. On the other hand, I hate watching anime with lots of stupid, frivolous-minded girls fawning over one guy. In the case of Clannad and Kanon, however, I was able to tolerate the girls because the main male characters are hot anime guys.
In conclusion, there is really no contest between harems and reverse harems in my book. I would choose a reverse harem any day simply because there are more hot anime guys in it. Hehe... ;)