BR, 4 burst kill, little aim assist.
DMR, 5 shot kill, moderate aim assist
The only advantage the DMR has is the aim assist. Otherwise the BR is better in every way
the BR fires in bursts at a lower rate of fire, you can miss individual shots within the burst and mess up your kills. Additionally, the BR has a shorter effective range, has a smaller clip size (3 round burst, 36 rounds per magazine, 36/3= 12 shots, DMR has a 15 round magazine), and a higher time to kill, from what I can remember. It is an objectively inferior weapon, but much easier to use given its larger reticule, increased fire rate within the burst, burst capability, and general flexibility. Ease of use: DMR < BR. Effectiveness at range: DMR >>> BR. Versatility: Depends on player skill. For very good players, DMR > BR. For decent players, or in chaotic situations, DMR < BR.