I tried in vain to find Portabella in an Italian dictionary: this seems to be one of those not Italian "Italian things", like the Alfredo sauce - totally unknown in Italy.

I also couldn't find it in mine, and that really annoyed when I first got the dictionary, because I was hoping to put that argument to rest for once and for all! Personally, I like the sound of portobello better. Maybe I heard it before I heard the other variations. I also noticed when reading through my "Italian cooking encyclopedia" yesterday that the pasta which is Alfredo in English is just something like creamy pasta or pasta with cream (can't remember the name exactly) in Italian. No mention of Alfredo, the mythical inventor of it.