Visited Iguasu Falls both Brazil and Argentina sides in Feb 2011 to compare both. I think both has their own highlights so since you're already there, you should visit both sides to get perspectives of this magnificant falls.

Argentina side takes a longer time to reach starting with a long bus ride to reach the entrance, then take the small train into the few stops and then walk almost a km to reach the Devil's Throat which is definitely the highlight. Then u trek back same distance to the train station n continue back to the Upper and Lower Falls area whr you will see countless falls and on a good day numerous rainbows.

There's an island in the middle that was supposed to be able to visit but it's closed due to high tides and strong currents (hmm, when is it never?)

Anyway personally I would prefer Argentina side due to it's varieties of falls (you name it, you see it) and the upper n lower gives you both perspectives of the falls plus the mother of all falls which is the Devil's Throat.

Word of advice: Wear your swimwear and anything waterproof. It's great to run towards a waterfall just above you! After visiting Iguazu Falls, no other waterfalls interest me anymore.