Hydraulic without question
Tandem cable lengths are so long it's hard to achieve acceptable braking on the rear brake. With hydraulic a proven and reliable technology on bikes, I can't imagine why you wouldn't make this choice. Unless you're planning on loaded touring down long descents where boiling the hydraulic fluid may be an issue. If you were brake overheating would still be a concern with mechanical discs anyway.
"Compressionless" housing offers an advantage, but it isn't really compressionless. It just has less compression than the standard, which is already designed to have minimal compression. That fails to address cable stretch. I'm surprised someone hasn't started marketing "stretchless" cables, which would be a misnomer anyway. Hydraulic fluid is indeed "compressionless," (for our purposes at least) although it sits within cylinders, lines and fittings that are not "expansionless." They expand minimally, but still expand a little bit.