Purely as a passer, Garoppolo is miles ahead of Brissett. Quicker reads, much quicker release, better pocket presence. Not a knock on Brissett, more a testament to Garoppolo's mechanics. Belichick has said before that quick reads and accurate short-intermediate passes are the most important traits he seeks in a QB, and Garoppolo, like Brady, excels at both.
What I think will be interesting to see is if Brissett can develop into a similar type of QB over the next couple years. Garoppolo had a head start on him, as he was the more polished passer coming out of college, and Garoppolo also has a 2 year head start on his NFL career, so this is all s to be expected. I think we'll learn a lot about Brissett's long term potential based on how he performs in training camp and preseason this year. It's possible that Brissett ends up like Ryan Mallett and, for all his physical gifts, never develops adequate mechanics as a quarterback. I think it will be very important to see if he shows significant improvement in the aforementioned areas, even in limited preseason time. What a player does with his first full NFL offseason seems to be the best indicator we have of what kind of player he'll become: the guys who go on to become really good make a leap, and those who wash out or stick around as JAGs typically don't. If Brissett has notably improved as a quick-strike passer when we next see him in TC and preseason, then I think we might have found a really good QB in him; the sort that would make you a lot less hesitant about letting go of Garoppolo if they're not ready to move on from Brady.