As the two others have said, it is a substitute in certain areas. If you're training an individual Pokemon in a stat(Attack, in your case) having that Pokemon do 5 Lv. 3 Attack Super trainings would give it 60 EV points. If you horde trained, you could get the same amount of EV points with 12 horde battles.
Pros of Super Training
-Used to quickly train a single Pokemon
-Can be sped up even more with Double-Up bags or training bags
-more fun(for me) to train with
Pros of Horde Training
-Can be used to train as many Pokemon as you can fit in your party at one time
That's pretty much it
Since you said you only wanted to boost the attack of 1 Pokemon, Super Training would actually be faster that Horde training and definitely faster than trying to find a bunch of miltank
EDIT: If you do choose to Horde train, if your EXP share is on, then all Pokemon in your party recieves the same EVs so be careful