I had used kent and then swapped to Firtz. Fritz was good. Now I am just using IO purple and it works the same as fritz for the most part. Not quite as much growth as the 2 years I used Fritz. But I have only used IO for 4 months now.
I recently switched from Kent to Red Sea Coral Pro. I found the Kent to be inconsistent with Alk. The Red Sea is easy to mix, just weigh it out. So far so good
I didn't use Kent for very long but the short time I did I found the alk to be pretty inconsistent and is seemed to produce a lot of residue for me. I like ESV the best buts it's just too expensive. I use plain ol IO now and while the alk seems very consistent the mag and calcium are generally not but I don't really care anyway because I doctor up each batch as needed.
to be honest I'm not sure I've noticed any real difference between the three meantioned. Any difference would be based purely on anecdotal evidence.
for me me it comes down the price, convenience, and the fact that people have been growing amazing reefs with IO for years and years. Plus you can get it petco or petsmart in a pinch.