I have the Supermew XX and highly highly recommend it for both tech kicks and fighting against ripping currents (like those in Indonesia). I've tried pretty much all the popular fins out there except for split fins, force fins (did try one pair but it was too big), seawing nova, and rk3. Except for Mares Avanti Quatro plus, everything else sucks big, big time compared to the XX. I can easily out swim every single other pair of fins except for freediving fins, and can catch up with Avanti Quatro. XX needs a bit more leg power than AQ+ but is a lot more precise for tech kicks. When I fly, XX is carryon size and AQ+ needs to be checked in. I can't handle jet fins and similar firm paddle fins in a strong current, but the XX cuts through everything like butter. I have chronic fatigue syndrome and fibromyalgia, I'm really not physically strong at all, prob one of the physically weakest diver out there. I wouldn't be able to confidently dive and manage other divers without the XX.
I'm very surprised your gf has the exact opposite experience. I'd maybe try AQ+, FF, Seawing Nova, split fins, and freediving fins if I were her. Many people swear by the RK3 too, and they seem considerably easier to handle than Jet fins.