I like Salifert because you can usually perform a lot of tests with each kit, but they are not refillable. I also like Red Sea kits, because you can perform a lot of tests, and they ARE refillable. They both work great, so I can't refer one over the other. They're not API cheap, but they're high quality test kits. Water parameters are your #1 priority, so you don't want something that measures "somewhere in the ball park". That said... API is okay for checking the cycle of a brand new tank. Other than that, hmm...
If your main tank has been up for 6 months (or more) and there are no big changes to the bioload, it should be stabile enough to only have to check for Nitrate. If you have serious algae problems, you might want to check Phosphate levels as well. If you have fish only, and maybe a few soft corals, these two should be sufficient as long as you do your regular water changes. If you have hard corals, you'll also want to check Calcium, Alkalinity, and Magnesium.
