I haven't used Rawtherapee much. I liked the creative opportunities it offered.

Darktable's workflow better met my needs at the time I was picking between them. In particular, Darktable was better for processing a few hundred images in a couple of hours right after shooting. Darktable's integrated image management was the key difference. Darktable's ability to offload processing onto GPU's was also a factor. GPU's make Darktable much much faster than without.