I prefer Seachem products for my calcium/alkalinity/ph buffering...but I've never used Kent products for calcium/alk/ph. Lots of folks seem happy with results from Kent products....but I'll stick with whats worked for me ......Seachem is readily available and easy to use. But thats just my personal experience.
I have used both and I definately like the seachem better. I found that it is easier to control pH and alk with the seachem stuff. The kent seem to have more filler then other products.