Dual chambered calcium reactors are advantageous because unlike a single chambered reactor, dual chamber reactors have a higher effluent pH which allows it to maintain the same calcium and alkalinity output. This helps to prevent a low Ph value in your tank.
Reverse flow reactors are nice, too, because they allow for more contact time with the reactor media. I think once you figure out what type of reactor would suit you best you'll able to narrow it down to a specific make/model.