Can somebody tell me what is the difference between these models for a give Sonic?

Different models (sizes also) and different years. Do you have a specific boat in mind?

I have just seen various 35/8 footers with these different suffixes. They are all '99-'03 models.

Lets see if I remember all this:
Diablo is the model name for the redesigned 358(like the 26' Prowler, 31' Hardbody)
SS designated the models as performance vs LS? that was more spacious
STS was an option package on the 38's

Or else I'm completely wrong .

Nope...your right stud! Let's go back to the RS! Now were talking history. The 31's are hardbody SS, 358's went Diablo in 01. The STS is a package for the 386.

Dude love that "inpropsicated" moniker but I don't think I'd be putting that on the transom
The water coppers would make their entire months quota pulling you over

Thanks.

There is no room to put it on the transom and meet USCG documentation regs. The only thing that has to be on the stern is the hailing port with the added on swim platform.

I have had the USCG safety check it and talked to various DNR police with no issue (knock'n on wood).

The joys of being federally documented.

I didn't read all the replies so I may be repeating this. SS and STS is the model designation ie;31ss not sure what year the boat picked that up. Diablo is a PAINT JOB, not a model, it's the name of the paint job.

Oppps dat's true...just that paint came out around the same time as the model change...stand corrected

Are you sure? Even the brochures read 358 Diablo across the top. You don't usually give the name of the paint job on the pic on the front of the brochure.