4 years of growth, larger earlier in a career, and different between prospect to prospect. I think 35 is probably the max growth in a category.
As for P vs. 3B (and the LF is out for me), I have a problem with the 3B "projecting" low 70s in vR, PWR, EYE. If he end up in the upper 60s, he's a major leaguer, but going off overall rating, he's gonna be way more expensive than I would be willing to pay for those numbers. If he were a switch hitter, it would be more palatable. The pitcher, velocity and GB/FB I agree are mostly meaningless, I'd be more worried about his pitches. Not that they're bad, just not great. But both guys are pretty close and I would probably base my final decision on the following that I don't see anywhere:
Is the 3B a switch hitter?
Is the pitcher 18, so using the high school projections?
What is the patience level for the pitcher?
If the pitcher is a high schooler, with a patience level over 70, I am taking that guy 10 out of 10 times. He's a number #2 starter you can control for 15 ML seasons at a reasonable cost. That is hard to find.
If the 3B is a switch hitter and the pitcher is a college guy, I'd probably lean towards the 3B.
If the 3B is RH and the pitcher is a college guy, flip a coin.