I've run the Korallin C1502, a no-name brand CaRX (probably an old PM CaRX), a Deltec P601S, and a MTC Procal CaRX. By far and away my favorite is the MTC Procal. It doesn't require a feed pump, has a secondary chamber to increase pH, has a flow meter, a needle valve for effluent control, and a pressure gauge to read reactor pressure.
My only complaint is it doesn't have a pH port for my probe but one of these on the effluent line with some PVC will do the same thing.
The Korallin ran brilliantly but was highly overrated by the manufacturer for the size tank it could handle. My 125-gallon was SPS heavy and it barely kept up, that's with a 400 gallon rating from the manufacturer. I think he could handle 100 gallons heavily stocked.
The no-name brand CaRX worked like a charm, my only complaint was it was a little undersized for my 215 gallon SPS heavy tank and I had to fill about 3" of media every month to keep up.
The Deltec is a POS in my opinion. I really don't understand how it was so touted back in the day but it couldn't keep a stable effluent to save its life. I think I'll use some concepts from the MTC and modify the Deltec to function better. It leaked as well and had to heavily silicone and use grease on the o-rings, then pray to the CaRX gods to keep it from leaking.
Like Dan said though, as long as it can keep a stable effluent, it's just really an acrylic tube. The carbon doser regulator he mentioned is amazing. I have one myself and wouldn't run another reactor without it.
Geo's have great reputations but I've never owned one and cannot speak about it.
With ratings, as long as it is rated enough for your tank, you're good to go. If it's rated higher, then just run less media or it just means you'll have to refill it less often. Don't get one rated lower obviously.
I believe Niko's has a Aquamaxx CaRX sitting in their used equipment room somewhere. I'd hit them up if that's the one you're interested in.