I have used MN48s in Thunder Tiger powered Q500 planes, and while it's the same size as the 3421, it's not quite in the same class. I'm not sure I'd use the MN48 sport servo in a Q40 plane.

Others may have more experience with them. I understand your thoughts, though - it's roughly a third the cost of the 3421 digital!

And if you're looking - I've used the Spektrum A5030 servo briefly - it's a low cost digital, about the same size. At 4.8V it was a nice servo, at 6V it was "nervous" when moving - didn't get smooth movement that tracked the stick, it wanted to jump around. I tried it with a regulated 6V, the local hobby store tried them with a fully charged 5 cell NIMH pack with the same results. I had my hopes up, but wasn't comfortable with the way they worked at 6V and above.

I am tempted to go back and buy a couple of them and experiment with regulator settings, see just how far above 4.8V I can go and still have smooth response.