We've been to all three (including PG). Blue-Grey in Lowell is an excellent forum to get in front of New England coaches and the price is low.($250 or $300, I think). Blue Grey staff is very helpful in assisting players in finding a good fit. While VaTech has been there, this is largely local. I cannot comment on BG's bigger events down south as we only attended one with my older son long ago before I knew what to look for from a showcase.
Top96 is run like a camp, which allows more direct contact between players and a subset of coaches. There were a lot of coaches there from a wider geographic area. Also key, many of the top local players attended. Price is 2 to 3 times higher.
PG Northeast Top Prospects is the biggest of them all, both from the perspective of participants and coaches attending. In my opinion, the PG format provides the greatest number of opportunities for a player to perform in front of the greatest number of coaches/scouts. Cost is comparable to Top96.