I was at oxora for the first time this year and since there was over 30 000 people there it wasnt that much smaller.
I would say that the scale of both festivals is about the same but since boom has alot more places with stuff it get´s about double the size.
Ozora was actually quite crowded and getting a private spot was impossible
But in the end i think it comes down to what you wan´t out of a festival, if you go for the music ozora def has the better lineup usually, shpongle kicked ass and there was almost always good music on the dancefloor and in the chillout.
Boom on the other hand usally has a few really good acts but fill out with alot of crap aswell, they claim it to be unknown gems but alot is garbage trance that noone seemed to like.
Although the 2012 lineup looks really good and as long as you know what you wan´t to hear it´s no problems at all.
ALso i prefer ozora´s soundsystem since i think function one sounds plastic, some music sounds really good like the hitech trance but forest sounds loose their organic vibe and the bass feels to amplified.
Ozora rocks martin audio which most good dj´s prefer since it sounds more hifi than PA which means better dynamic and less unts unts.
But overall boom still has alot more to offer since at ozora we got preety tired of just dancing and hanging at the chillout all the time.
So it depends if you are a dance floor fiend i think ozora is your best bet, but if you wan´t a more filling experience and have more cash to spend then go for boom!