Berrios has way more college experience as a receiver and as a punt returner than McCarron. IIRC, Berrios also played some on ST in a non-returner role.
McCarron has more experience in the Pats' system. He became a permanent fixture on the 2017 Psquad/scout team as of last September 22nd and won multiple Practice Player of the Week awards. He's now also had the full course of OTAs, plus 10 days of Camp. Based on college highlight reels, McCarron also seemed more agile and explosive than Berrios.
I'm guessing that both will see significant snaps tomorrow night which, hopefully, reveals something about both.
Speaking of tomorrow night ...
Since the booth guys ordinarily do an extraordinarily crappy job of correctly identifying the player on the field during pre-season games (when they even bother to), I have to wonder if the current rookie "numbering system" will still be in place to screw things up even further, or if the team will assign "real" numbers sometime today.