They are different playstyles, the DSG is a powerful Rifle, its unforgiving, hyper accurate but requires user accuracy, to add you must be stationary while shooting; the Takedown is a DMR, the permits running around at will, but it requires 2 shots to the head, and has incremental recoil and requires the most discipline in the game, yet is one of the most forgiving guns when it comes to ammo.
I wouldn't say i would pick one over the other, they are situational, the DSG is amazing if you have a team backing you up, but performs poorly when out number, due to the fact you cannot move.
The Takedown is a weapon that can wipe out entire squads, so long as you are disciplining yourself.