Actually, there are 3 D back ends. There's the original Digital Mars C++ backend for DMD, the Gnu Compiler Collection backend for GDC, and the LLVM compiler backend for LDC.
Each has their strengths and weaknesses, and some users even switch back and forth depending on what they need at the moment.
I was keeping it simple as the DM backend cannot compile (say) Rust whereas LLVM (and sort of GCC) has frontends for both so you can make the comparison.
I myself am not in the "move to ldc" camp, I like having dmd available for speed.
Each has their strengths and weaknesses, and some users even switch back and forth depending on what they need at the moment.