I see how you would want to do the "Trade", and I'm not trying to rain on your parade, BUT you are still going from 1 Fountain to another, correct? What happens if the 03' 35 has problems? Do you think you'll have any better customer service from Fountain? I remember your horror story about you trying to get Fountain to fix your 42!! BTW, Fountain owners, I'm not trying to bash Fountain, I'm just reciting this man's story.
I know alot of what the trade will do for you is get rid of that "Is it going to break every time I take it out" feeling, I just hope the 03' is a perfect boat, and you don't have any problems with it.
To answer your Q, if it were me, I'd keep the 42 IF, BIG IF, it all is straighten out! 83-84 MPH in a 42 w/HP500 EFIs is pretty darn good, I think! The 42 is a much more sea worthy hull, and the way the Manatee thing is going here in FL, I'm afraid, here in the so-not-distant future, the OCEAN is going to be the only place to run your boat, over 5-25 MPH that is!
Good luck, and if you ever head down to Port Canaveral, look us up. We live on Merritt Island, off Sykes Creek.
PS, I had Mike @ Hall's lab my 28" bravo1 and picked up 2-3 mph!