The point remains that regardless of the case, the airsoft gun isn't being allowed in the first place.
Obviously, some people don't have a choice (when I started airsoft I didnt' have a car).
Now if you choose to break that by-law and take it upon yourself to face the possible consequences, a guitar case is much less likely to draw attention than a gun case. I rode the subway plenty of times with camo pants, combat boots, a heavy metal tshirt, a backpack and a guitar case and nobody gave me a second look because the "look" all fit together. Don't put Magpul stickers and shit on your guitar case like a fucking idiot.
By contrast, if you're dressed casually with a locked gun case and looking around nervously because you're afraid someone will notice, you WILL draw attention to yourself.