I looked it up some time ago, and came to the conclusion that the correct name for the usual mushroom we eat in the West (unless you're in England, in which case Field mushrooms could be on your plate) is portabella. What we often see, of course, is portobello -- the masculine form of the same word. I happen to prefer portabella, perhaps because that's how I heard it first (perhaps because it's correct?). But 'portobello' is at least consistent, as you'd expect an Italian word to be.
What I wonder at is the linguistic ignorance of people that mix and match: 'portabello' or 'portobella', which is like handing a monkey an unbrella instead of a banana: they just don't go together. A word is either masculine or feminine in its components: you can't have it both ways. Though in our anything-goes culture, I suppose a lot of people think you can!