Actually, it might be easier to just present my findings from my notes on the both...nerdy, I know but I do keep track.
Both hold properties close to the description given by Tropic Marin with my findings being;
The values are based on my measured salinity of 35 PSU and had to use right at two cups and a quarter per 5 gallons mixed with my RODI fresh at the same temp as my DT.
Classic was right at Ca: 380 ppm, Mg: 1250 ppm, Alk: 9 dKH
The Pro reef was right at Ca: 440 to 450 ppm, Mg:1350 ppm, Alk:8 dKH
The pro mixed with a pH of ~8.3 after a day to bounce to true while the classic bounced the day after mixing to a pH of 8
The classic mixes faster (which I understand) and completely clear whereas the pro took longer and appeared somewhat cloudy with some dark residue around the rim of mixing container and a little in the bottom which I did not like and was cloudy in the DT for up to an hour before clearing.
Other than mentioned I noticed no significant differences.
Hope this helped. This was just IME but I know 12 to 15 years back when the pro was called premium it seemed to mix better and my coral did really well. Kind of makes me wonder ???????
So...I am switching back to the regular classic.
EDIT: had to correct measurement from gallon to per 5-gallon mix...ARGH. Sorry 'bout that