both will maintain calkcium and alkalinity but in a different way
on is cheap the other is expensive to buy and they are both relativaly cheap to run
I have supply my tank with kalk reactor for a long time and find it a cheap
and easy way to do it with a litle maintenance IMO they are a realy good add for beginer tank over the calcium reactor since they will climb up the PH and precipitate phosphate and this will protect you a bit from the nasty algea bloom but there way to do it and you should do it right like any method cause if you do it the rong way you will scew you tank like any other method
For the Calcium reactor I just bought one last week but still not install on the tank I will anyway use both cause I dont want to stop using kalk other could tell you more about them Im not the expert on those
you also have the 2 part product but they cost realy to much on a tank with medium demand and recomande them only for small and low demand calcium another ting I dont like from the one like bionic or ESV is that they add all kind of oligo and metal who are useless and IMO more detrimental over the long run
Here a good link you should read to know how calcium and akalinity work
http://www.advancedaquarist.com/issues/nov2002/chem.htm