Salt selection should be tailored to your tanks specific regime. There is no "right" answer to the "best" salt, as depending on the type of your system (FOWLR, SPS dominated, LPS, Softie, MIxed reef) any of the major brands of salt could be the best choices. Other factors to consider is your dosing amounts, water change volumes and frequency of water changes. Here is a post I just read recently on another forum and it is really bang on for some of the reasons people prefer DD vs. RC or IO (or RC/IO vs. DD).
"I've been using H2Ocean for several years now in my reef tanks for the same reason - it's derived from natural seasalt. I add Mg and K to H2Ocean and my testing has been very consistent with it. I used IO (and IO Reef Crystals) almost exclusively from 1993 to 2009 and I thought I was happy with it until I switched to H2Ocean. When I used IO I added Ca, Mg, and K. In my opinion, for the average reef aquarium IO is a perfectly fine salt to use. If your tank is PACKED with SPS corals (in particular), I think IO is sub-par. Having said that, I've seen AWESOME SPS tanks using almost every brand of salt out there. I don't think salt choice is the be all end all as long as you stick with one of the more major brands. In the end, I think it's more about parameters. If you want to keep alkalinity at 7-8 dKH, IO or IORC is a really poor choice because they have high alkalinity. If you want 9-10 dKH, then H2Ocean is not the best option."
I pretty much agree with everything this poster has said.
