I've had both.... I've removed the nitrate reactor from my tank and just running a biopellet reactor.
Nitrate reactors work but it takes a lot of effort to set up the drop rate plus it does drop your PH down. Also I had it setup on my 265g and considering it just slowly drops water it takes forever to cycle the tank and considering it moves so slowly I personally didn't notice any difference in the parameters of the DP.... I just found it a waist of money on sulphur.
The biopellet though is amazing.... best reactor I've ever bought. It really makes a difference, haven't noticed any drop on alk
or anything but it does maintain nitrates and phosphates. I guess maybe the alk
drop might be from cheaper branded biopellets... I dunno..... but I use reef octopus and never had a problem