Should I use Ditra for my slate floor with radiant heat and 2 layers of plywood?
Gypcrete is about 27% lighter than concrete (http://en.wikipedia.org/wiki/Gypsum_concrete). If it's not too late and you don't mind tearing up the second layer of plywood, the gypcrete will be a LOT more favorable in every way. Better for heat transfer. You can tile right over it.

That said, if you're already set on the path you're going, yes, I would use Ditra. You have to use something (a lateral anti crack membrane like Ditra, or hardieboard etc.) Ditra comes highly recommended as apposed to hardieboard. There are also Ditra alternatives that are the same basic thing (lateral anti-cracking underlayment.)