This is just my experience with marine mix...
The salt has a more consistent, and finer grain size (more like a powder) than the reef crystal salt, it has a higher concentration (1/3 cup per gallon vs. 1/2 cup per gallon to sg@1.025) It leaves less residue when mixing, and dissolves faster from my observations. From comparing test results between reef crystal and and marine mix I also observed slightly higher calcium, kH and magnesium levels with the marine mix, more suitable for a reef tank.
All in all thou, the reef crystals is a good salt, I just recently finished a bucket of it myself, but since switching to the marine mix my reef tank just seems happier. It could be coincidence, or maybe not. However, for the price difference, even if they are scientifically proven to be equal salt mixes, I will take the savings of the marine mix when it is available for around $30/box. There is an overall savings compared to the 200 gallon box of Reef Crystals when you compare price per gallon, and paired with my personal observations above, is what makes me say it is way better than reef crystals.
Although, no two tanks are the same, so, YMMV....