When swimming against the current for whatever reason, I would say that the Quattros use less muscle power per kick, but the Slipstreams give more boost per kick.
One of the main reasons I decided to try out a stiff paddle type fin was finding myself in situations where I have needed to swim against the current (getting back to the anchor line, working thru a strong down current, etc). The Quattros and other fins I had used had a point where I gained very little or no ground despite pushing the fin to maximum effort. The Slipstreams stand out as being better for me in those situations.
For a one type fin to fit all conditions, I have grown very used to the Slipstreams and comfortably use them for surface skills in addition to dives. If only doing surface work and not diving with the potential for stronger currents, I could see a preference for Quattros. They are nice on the surface.
Yes, that makes totally sense. It's pretty much what it felt when trying them out. It's a tough choice.
Here in this part of Mexico I also have to do snorkel with whaleshark outings (diving is not allowed), and more courses than I'll be doing in Thailand later on (where I'll be mostly guiding on liveaboards for 6 months).
Buying slipstreams here would also be cheaper here than in Asia, but I can't keep both for weight/volume issues...