I voted "both," and did so with significant displeasure.
The problem is, I DO believe Advantage and Disadvantage should be special, reflecting a distinct and significant difference in the situation (or your training, or w/e else) that makes you substantially more(/less) likely to succeed. Say, a Thief with the Criminal (or Spy) background, making use of Inspiration by saying, "this lock? Yeah, I had to break dozens like this when learning the trade. If you have the wrong pins, it makes a slight click noise just before it jams--so you can back out before that happens. Easy as pie to crack, once you know 'em well."
But that's not what Advantage is. Advantage is supposed to be a weapon of last resort, something major and significant--but, due to the construction of the ruleset and the advice it gives, it has been turned (as I've said elsewhere) into a weapon of first resort, to the detriment of the game. It leads to this weird situation where Advantage (or Disadvantage) is the first and obvious choice for giving a bennie(/penalty), but then if things get better(/worse), you have nowhere else to go, because the system makes it one of the biggest benefits around. Expertise is the only exception, and (AFAIK?) only two classes get that, so it's seen as a Special Thing of Rogues and Bards, not to be shared generally.
So yeah. It's both Special (because the system was constructed that way) and Commonplace (because the system provides minimal support for anything smaller).