Superfly and Firefly are just different artistic media like oil paint and water colors are different media serving to the same end. Both use a brush and a canvas, but the paintings have their own signature. Both have their own techniques to apply color and both have things that are easy to do and things less easy. There is no matter of succession. You can even get the best of both. Make a render with both, overlay the results and rub out where you want to show the lower layer. 

Firefly has its strong points: microfacet displacement is a lot easier in the fake 'artificial' techniques FF is based on, but notoriously fidgety in the physics based processes of SF. Also there is a strong base of legacy content that is based on Firefly. I think Firefly will stay there for a few generations of Poser. The sketch designer is still there also.