The main difference is the "projectable" part. When scouts speak of it, they talk about "present" power and a projectable power "ceiling". Youkilis is a guy who confounded scouts because his body type lent itself to producing more power and he eventually grew into his projections. But the truth is, that if it isn't there, it just isn't. Look at guys like Daric Barton, who looks like he should have some big power numbers, but they just can't generate enough force to hit for power consistently.
The difference between power and plate discipline, although they are both tools , is that one is purely physical. Power, like speed, cannot be taught. If you want an example of a guy whose approach refined to the point where he went from a hacker to one of the league-leaders in OBP, P/PA and walks, look no further than Edwin Encarnacion. There are other examples of guys improving their approach like this.
I want you, however, to find me an example of a guy whose power ceiling was projected to be below average and ended up hitting 30+ homers without some "help". For the record, Ellsbury is not an example of this, since he was always projected to put up big power numbers, but his approach never allowed it until 2011, and the multiple injuries have stunted him since.