The Gulfstream is a shorter Sailfish. Cabin suffers the most vs. Sailfish.
The Gulf just like the Islander can work on a single motor option, but same issues in towing legally as Sail.
And a Marlin is a scalled up Sailfish in all dimensions, with a integrated transom from day one. No single motor option on this baby ! (gas use 25% more than Sail)
That's the whole shebang in a nutshell.
Narrower beam Islander should be more fuel efficient given equal weight boats same LOA & hull V.
Ride has to do with deadrise and the V of bow forward, and more importantly, to what each of us defines as "good ride". Don't make the same mistake as others, ride it for YOURSELF in the wind/water conditions to encounter in YOUR USE.