I use a combination of both. Basically, I use Seachem's dry stuff for basic chemistry and Kent for more "exotic suplements" and some food. I prefer Seachem's dry to Kent's dry because I get better results, especially with the buffer and calcium. I have used Kent Tech CB before and had decent results. But I never got better than when I moved to dripping Kalkwasser (use any brand you like or pickling lime from the store, they're all the same).
Currently I drip Kalk, bump Ca with Seachem calcium supplement if needed, buffer once a week, almost never touch the Alk supplement with Kalk dosing, dose Mg twice a week (need to with Kalk), and once a week for all the rest except trace and phyto. Trace goes every-other (mainly as an insurance policy probably not necessary, but I have the stuff) and phyto goes in 3x a week.
Buffer, Calcium (Reef Advantage), Alkalinity Supplement (Reef Builder), Magnesium (going to switch to Epsom salts for that for $$$ savings), Stronium
Iodine, Coral Vite (vitamins), Trace elements, Phytoplex (for the clams).
Last I checked, calcium carbonate is calcium carbonate, and same goes with any basic chemical. I'm pretty sure that those CaCO3 molecules don't know whether they'll be put in a SeaChem or Kent bottle in the end. I believe most basic chemicals come from the same places and distributors (most CaCO3 comes from dragging the Great Salt Lake in Utah).
Having said that, the companies may refine the products more or less. Other products will be different, such as carbon (different materials) or CoralVite (or whatever supplement you may choose).
In short, I don't think you'll see a difference in your basic chemicals when choosing from either company. My theory is to stick with one company for everything - it's unlikely that SeaChem and Kent test their own products against their competitors for interactions.
I personally use SeaChem because I know and trust our SeaChem rep that comes and talks to us - usually once a year. He certainly knows his stuff and can answer any specific question you may have about any of their products in depth. Thus, they get my support and dollars.