Henry
Autoexreginated
Of course, at least as I see it, a simple +3 would be potentially much more problematic because it could stack. Even making flanking a +2 is probably going to have more impact than advantage, at least a lot of the time.
From what we've seen, it would make the most difference toward the extremes -- e.g. if you needed a 20 to hit. A +1 would be about the same odds, and as your target number was closer to 11 on a 20, it would make less and less difference than Advantage would.
Given the bounded accuracy of 5e, and having used the advantage flanking option from the DMG for an entire campaign, I'd be more comfortable with a straight +1. Advantage meant that EVERYONE wanted to gang up in combat -- PCs and enemies. I will admit though that the players would figuratively lose their water whenever they faced against a horde of CL1 or 1/2 creatures in close quarters, even at high level - because they knew if they didn't think tactically they could be nickel-and-dimed to death...