Reefapalooza
•tons of livestock
•some equipment
•some speakers
•more laid back
MACNA
•lots of livestock
•incredible educational opportunities
•run like a serious convention, including sightseeing trips and usually something associated with local public aquarium
•virtually all equipment manufacturers, with latest new launches
•formal program including workshops
If you have the time and money to do just one I would say MACNA is it. If you participate in the talks and workshops and extra activities it is a chance for total immersion in reefkeeping for 3 days and definitely a truly memorable experience. I have been to and exhibited at both, more MACNAs than Reefapaloozas.
Ultimately, it depends upon your desires, experience level, and desired outcome.