Technically, or rather temporally, AQUA comes before ARIA. However, its release date fits in between ARIA volumes 3 and 4. I think the right way to look at it is to consider it as a side story for those who want to know more about the characters.
I actually find AQUA easier to follow because it follows the progress of time more strictly. Even so, I think one should start with at least the first 3 volumes of ARIA because part of the fun for this series is its temporal aloofness.
By the way, I didn't know since I watched the first 2 episodes of the anime before reading this, but ARIA the Animation includes stories from AQUA as well.
