Sonora, welcome to the forum. I can see where the dealer is coming from, but plfinch makes some very good points with his excellent first hand experience.
The Outfitter is not a fast cruiser and oddly, can be slower in the tandem mode than solo. For instance, I can travel 4.7 miles in an hour, solo with ballast (GPS, no tide or current, two directions), whereas, my wife and I can only do about 4.4 miles in one hour together (same route) -- the heavier bow wastes significant energy as it plows a lot of water. In that situation, the extra potential power of the Turbos becomes virtually wasted once the boat "hits the wall" with two people. You can go almost as fast with considerably less power! When operating solo, on the other hand. the extra torque available with Turbos produces good results.
The Oasis is another boat that is hull speed-limited. A friend and I managed to sprint 6.8 MPH with two sets of Turbos; the wife and I can get the same speed with one set of Turbos and one set of STs. As a comparison, we can move the Oasis about 5.0 miles in a one hour cruise, tandem or solo (same route, same fins).
ST's are an excellent fin -- They're almost as easy to operate as standard fins, yet (unlike the standard fins) they can produce almost as much speed as Turbos (but at a higher cadence). Additionally, they cost less and are a no brainer to install.
I'm a big fan of Turbofins, with their slower cadence and quicker acceleration (they also work better as a centerboard when sailing), but have a lot of respect for the STs as well. If you're running strictly tandem with the Outfitter, I think the STs would give you virtually the same benefit as Turbos. It depends a lot on personal choice.
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