Hi Giovanni,
Funny you ask that, as I'm working on that very piece right at the moment. I never learned it before, so I never went over editions of it with my teachers. I have an old Kalmus copy, and I'm not sure what that is from. . .is it Peters? And I just got a copy of the Baerenreiter. I don't know a lot about these editions, but I think what I'm going to do is see if Westminster Choir College has a copy of the Neue Mozart-Ausgabe, and try to take a look at it. If the NMA does have it in the 4 stave version, I'd like to make comparisons. This, I think, would be the best way to go, because I would like to know what I'm dealing with in these editions.
I'd say that I like the Kalmus edition for the outer sections, and perhaps the Baerenreiter is a little more complete for the middle, but I won't really know until I compare to NMA.
From the standpoint of just playing from the editions, I find the Baerenreiter seems simpler, and yet I find it more difficult because the voiceleading makes less sense to me than the more complicated Kalmus edition. It seems that if the music makes sense to me, my fingers will find the notes.
I can't give you a strong opinion on this yet, as I'm still forming mine. But perhaps someone else can give us their thoughts on editions, and perhaps I'll add to this after I've studied some more. I expect I'll probably eventually make up my own edition in Sibelius.
