Grullon is all of 22 at Reading, with a .822 OPS and H/A splits of .800/.844, so he's not benefiting from the Reading park. Combine that with above average defense and you have one of the best catching prospects in baseball.
Bossart is a smaller sample, but he seems to be blossoming late, as his power has made a big jump, and again, the H/A splits suggest it's for real: .834/.911. While he's older at 25, he's also a solid defensive catcher with a reputation for handling pitchers, and he wouldn't be the first catcher to learn to hit later than normal.
Both are certainly worth watching, but with Alfaro entrenched, I expect them both to be promoted to Lehigh next year.
If they continue to do well, catchers are really nice trade chips.