I have both rostered and am struggling who to start today.
Jesus seems to be consistently mediocre, but there is hope that as he matures, he can turn it around. Teams have better scouting reports on him now and are exploiting his batting weaknesses. He has struck out 24 times, while only walking 2 times in June. Compare that with 23-10 in May. If he can adjust and turn that around, then there is a great hope that the power will return. He is only 22, after all. More troubling though is his right-lefty splits. He is batting only .209 vs. righties. Since the league has more RH than LH pitchers, the outlook is a bit bleak. he is also ..24 at home vs. .279 away, so there may be a bit of a penalty for playing in Seattle. given that the month of June was spent at home and 9 games away at pitchers parks (LAD, SD, OAK), that might be greatly screwing June results.
On the other hand, Grandal has insufficient stats to determine how he will fare against MLB pitching once teams get accurate scouting reports on him. If his PCL record is any indicator, he wlaked more than he struck out, which is highly unusual for anyone. This is evidence that he has great plate discipline and should adjust to MLB level pitching. However, Grandal doesn't have the same power that Jesus has. So, I suspect that in Grandal, we have a batter that will have a high average and not much power.
consider Montero's minor league HR totals in 2008- 2011 (17, 17, 21, 18) and SLG (.491, .562, .517, .467) vs Grandal 2011 totals (only full year in the minors) HR-SLG (14 and .500). Clearly Montero has more power potential and more experience. Montero has less downside. Grandal has some growing pains to work through and his upside is really unknown.
So, I would not drop Montero for Grandal, if you have a bench spot to hold onto Grandal, he might be worth having on a roster, perhaps starting Grandal in days/weeks that Montero is facing mostly RHP. at least until Montero can make adjustments and get his AVG against RHP back where it needs to be.