I don't know squat about this stuff but here's my ignorant reaction:

If the Datrex stuff isn't making you thirsty, that means it already has its own moisture, which means it's not maximally dehydrated and therefore doesn't have the highest possible number of calories per pound. That means you need to carry more of it to last for a given amount of time, assuming you have a source of water. So, that's a reason to think Mainstay might be better.

Also, the Mainstay stuff might not taste good, but in a survival situation you won't care. If anything, tasting bad will make it last longer. You're supposed to have just one of those little cubes a day or something like that, a bare minimum to stay alive. The bare minimum means you'll be hungry all the time, just not dying of starvation. The temptation to use up your supplies faster than you need to will be great, so tasting good is not a plus.