I did find with Tested Power2 x 470-hp MerCruiser 500 EFI :

THe Formula 353 wiill run 74.4 mph

The Scarab will run
454 MPI 71 mph~
502 MPI 76 mph
500 HP 82 mph

It is hard to believe that hull difference is 7 mph on boats that weigh within 200 lbs.

Thoughts.
600 DPX 92 mph

A 353 with 500EFI's will run 80mph. With the Stelling ext box option, they will run 82mph.
Another friend had a 33AVS with 502mpi's. It ran 76-77mph and then he added ext boxes and it ran 80.

I am only going to comment on fit and finish of the two boats. I have owned 3 Formulas and now a 38 Scarab. There is ZERO comparability to the build quality of the Formula to the Scarab, Formula being superior. With that said the 38 Scarab that I bought, is one of my favorite boats ever made, so I'm willing to do whatever it takes to bring it up to the quality of a Formula or Cigarette for that matter. So if build quality is important, go with the Formula, but I've been told by many people that the bigger Scarabs are hard to beat when it comes to handling and speed. Like everything, it's always a compromise.

That 82 mph 33avs/500 is with std 30 pitch props, about what mine runs w std 30s, i run 30 labbeds and see 84.5 gps or so heavy and 85 to 85.2 running light, stock 500efis w rev limiters turned up 100 rpms. Fwiw, smitty

Hated interior in my 33avs, sold power bolsters, had mech bolsters built, added drink holders to back of them, made back seat much wider, split engine hatches like a fountain, million times better! I think im going to tear out sink, toilet, cabinets, etc and make a large, comfortable bed in cabin next!

Mine has the side by side bolster. I have a set of power Mcleod power bolsters out of a '99 353 that will be going in the Scarab. I've got saddle tanks in my boat, which I am trying to decide to keep to remove. If I pull them, I will gain a TON of storage space, but then I will lose 130 gallons of fuel capacity. Right now, I'm really leaning on toward leaving them in.

AS to the OP question, it really looks like you need to decide as to what is more important to you? Speed, or build quality/features/comfort. One thing to remember, you can always improve the quality of things in the Scarab. With the Formula, you cannot really do anything to the hull design to make it faster, other than add more power.

Keep the saddle tanks. With those 1471's you'll wish you kept them unless your just cruising up the shoreline 10-15 miles. Plus I might need a life line cause I don't have them. Lol... Sr's came in handy big time.