I've had MainStay and Datrex.

Both taste great in my opinion, I ended up eating a whole datrex (3600cal) as snack around the house... it's light and flaky. tastes similiar to coconuts.
The mainstay was MUCH dryer (to me atleast, its spposeldy pretty moist, maybe due to the diff. texture/compound than datrex) (taste like lemon) and harder to eat without water.

With that said, I have a case of Mainstay at home but in the future plan to buy cases of Datrex instead. If you buy Datrex by the 3600cal packs it's cheaper than the other size that equals the same amount of energy.

I'd get a case of datrex 3600cal stick it in the trunk and call it good you could go a LONG TIME in a survival situation "waiting" eating them.