Which Robin?

While neutral is boring and has no boon/strength, I would prefer using it over the -DEF for a few reasons. As you stated it's a superbane, but also the problem is simply that at 26 defense she won't be in a position to fill the role I think she fits into best - and that's all-around colorless counter which no real weakness besides anti-dragon units. If you strip her defense, the degree to which you can use her in a myriad of circumstances decreases.

Dragons take on armors incredibly well because armors usually have a weaker magic resistance, and of course dragons are hearty and sustain damage really well, but put Robin at 26 Defense and she's not going to do that effectively. Neutral isn't a bad place to be for a dragon, and hopefully later on you'll get lucky and roll a more favorable bane (HP).

-def isn't quite as bad as people make it out to be. Without the bane she has 34 def on enemy phase and 40HP unless you take a bane. That's probably enough to take two hits. With the superbane you're now at 30 def, and you can maybe squeeze through with ally support. I would be more concerned about the superbane allowing opponents to knock me out of QR. A lot of units have atk around 50, so with 34 def if I can get a little ally support I can sometimes limit damage to 15 and keep QR. With 30 def, significantly harder unless you rely on fortify fliers.

I would recommend +atk -def for a desperation or vantage build and neutral for probably anything else. Personally I think I would prefer neutral because goad fliers is so powerful I may as well keep my def. I might not need the extra atk a lot of the time.

Keep the neutral. That way you won't be lacking in the defensive department, something Robin really appreciates, and you'll have all around good stats to boot.

On the other hand, you can try merging the -Def one into your Neutral Robin, unless of course you REALLY need to give another unit either Cancel Afinity or Res Smoke.