Yep, it's exactly the same.
Say you use a Speed Bag S on a Pokemon. You gain +1 in the Speed cat., or one Speed EV.
This is the equivalent of gaining EVs from one Magikarp.
However, Super Training is much more efficient.
Say you use a Speed Bag L on a Pokemon. You gain +12 in the Speed cat., or twelve Speed EVs.
This is the equivalent of gaining EVs from twelve Magikarp.
Source: Experience