I prefer Firefly. I sort of fell in love at first sight with Firefly. It was such a huge improvement over the P4 renderer. I was one of those who spent hours adjusting the old MAT files to take advantage of Firefly. (I happened to come across some of them recently, and boy, do they look terrible. Not sure if Firefly changed, or my aesthetic did, but they're baaad, LOL.)

I don't particularly want to do photoreal renders. My style has always been more illustrative than realistic, and I'm more likely to use Photoshop to make a render look hand-drawn than I am to try for something that could be mistaken for a photograph.

I also found Superfly really slow and noisy. That's been improved in Poser 12, but I still prefer the look of Firefly. It seems clearer and sharper, somehow.

I also find a lot of Superfly renders fall into the uncanny valley for me. I think part of it is that some things can look SO realistic, like skin and eyes, but others are very difficult. Like hair. It either looks painted compared to the rest of the render, or it has a really plasticky, Barbie doll look.

Firefly, to my eye, has a more similar level of fakeness for everything, so is less jarring to me.