I would suggest you study, very carefully, the four ergonomic diagrams of the four Spyder models posted by Bob (above). This is critical information.
The differences in riding positions between the models are very well detailed here. How far you lean forward. What kind of knee bend there will be. How much of a hip angle. How low you sit. Hence the RS is Sport, ST is Sport Touring, RT is Touring, and F3 is Cruiser, each with a unique riding position.
The RS leans you forward more than any other. The RT sits you upright. The ST is in-between these two. The F3 is the most different, it sits you much lower and your feet are out in front of you - cruiser riding position. Some riders LOVE this riding position. I personally found I was too tall for the F3, despite the superior handling it provided from the lower cg and shorter moment arm. My knees were constantly hitting the panel edges alongside the fuel cell, and the wind protection (lack of) was simply unacceptable for how and where I ride.
So, your physical size is a factor as well as your personal preferences. Personally I fit the ST better than any other. You will likely find one of them to "fit" you better than the others.
In an ideal world, you should sit on each model. Better yet, you should ride each model. But, try to find a dealer with all four models available for a test ride. Good luck.
For some, the three-cylinder 1330 ACE engine is reason enough to buy an RT or F3. It is not available in an RS or ST. I found the six speed tranny of the F3 to have the gear ratios way too tight, all I was doing was shifting. With that kind of torque, the ratios should be spread out further, not tighter. Ideally.