Under sink mounts work fine...
90% of the RO/DI systems use the same type of cartridges, same form of canisters, etc. Go with those. Buying something proprietary is going to cost you in the long term.
I've been buying from AirWaterIce since long before BRS existed. They stand behind their gear, refills and add-ons are reasonable, they're easy to talk to, should you have a problem... I just don't see a reason to change.
Their Typhoon 5 stage, at $159, is an incredible value. Comes with everything you need to start your reef right, with zero TDS water.
I started with their Dual home/reef system, added a 3 stage DI system, a pressure booster, permeate pump, and changed out the inline carbon for a 2nd full size carbon filter. That system now feeds my reef, two refrigerator ice makers and drinking water systems, and a drinking water faucet at the kitchen sink. Put it all in a narrow cabinet on the wall behind our kitchen sink. Easy access, nothing crowded... couldn't be happier with it.