From the outside, the cabs are the same, so they look similar. However, the U-series is for highway use, where the DM is the severe duty model (think dump, concrete mixer, etc.).
This might help, too (1978 DM brochure): https://public.fotki.com/modeltrucks25thscale/truck_brochures/mack/mack-dm-1978/
No brochure for the U-series.