Hi,
I can not vouch much for the Aqua Medic reactors. We just started carrying them, but I have not had the chance to hook one up and check it out yet. The Geo, Precision Marine, and Korallin are all great system in my opinion. I would rate the PM and Geo a little above the Korallin just from a "setup" point of view. The PM is extremely "set it and forget it". The Geo require just a bit of tinkering at first, but settles in nicely, while the Korallin seems to require the most fiddling with. One thing I like about the PM over the other two is the flow direction in the reactor, where the water flows from the top down, the others are bottom - up. I think the top down provides better water movement through the reactor, causing it dissolve more efficiently. In the end you probably can not go wrong with any of them, and will definitely be happy with whatever choice you make.
The ph is the main factor on how fast/slow the media dissolves. I have used Geo Marine crushed coral, Knop Koralith, and CaribSea ARM. I like the ARM the best, as it melts at a higher ph (around 6.8-7.0) The others need to get the ph down around 6.5 - 6.8 to melt efficiently.
Thanks,
Jeremy