I've used the HM Digital PH and TDS pen style meters, and both worked great. Until I let the PH meter sit for several month and dry out. Rehydrated it and seemed to work after being re-calibrated, but then switched to soil and haven't done hydropoinics grows in a while so it's probably dried out and dead again now. :smoking:
I used a Bluelab Guardian for a while but didn't like it as much as the pens. If you're just going to run the Guardian probes in a single res, and not move them then it would probably be a much better experience than I had. The PH probes in particular I found to be a bit on the fragile side for being frequently moved. Personal opinion only, YMMV.