I've owned:
MRC:broke before I could use it, not impressed w/ build quality. However, to be fair, I bought it used, so keep that in mind.
Korallin C1502: Giant PITA to get dialed in and keep running with the fine media. This reactor is designed to seriously conserved CO2 by recirc'ing it hard core and keeping the internal pH pretty low. Small media dissolves quickly in this sort of environment and turns to mush easily, thus clogging the reactor. I dont think you'd have the same issue with large media, but I had decided enough was enough, and sold it to buy a Geo 612.
Geo 612: NO complaints. No really, none. I now have two of them. Doubt I will buy anything else besides a Geo. It takes longer to set up, than to dial in...30 min max.
Nick