I've had (1) Flat Deck and (2) Mistress's. The Flat Deck was an '83 with 482s which was a stock Merc. engines with TRS engines. VERY VERY heavy boat. Probably ran 58 MPH with the wind behind you and low on fuel.
I had one '85 Kevlar 35' Mistress and one '81 fiberglass. Big difference.
If you don't want to go fast, have a lot of money for fuel, and need the extra room - Go for the 38'.
If you want to go fast and don't need the extra room - go for a 35' Mistress. Go for the Kevlar if you can find one. They are stronger, lighter, and more efficient on fuel and probably 7-8 MPH higher for the same power which, in turn, costs you less to go each mile.
As you can see, I've had lighter and heavier boats. I prefer lighter for a lot more reasons. Ask anyone who owns a Cigarette.
For 35k more you can have a 1991 Carbon Fiber Top Gun with 525SC that runs 74 MPH.
I hope this helps.