There was a website where a guy compared 7-8 different programs that do interpolation- but I can't find it right now-- but SI Pro 2 came out just about on top as I recall- and it was among the CHEAPER methods. It uses what is actually a somewhat well known (among Photoshop wizards) of stair-stepping an image up- but SI Pro 2 automates the process.  I tried it along-side of a full working demo of Genuine Fractals- with a variety of different image types- and I could NOT see hardly any difference- but that SI Pro 2 seemed to have the slight edge. Of course- SI Pro 2 completely wins for the price difference.  I think Genuine Fractals has its biggest advantage in doing EXTREMELY BIG blow-ups- like City Billboards- with a relatively smaller file size based upon fractal compression. --- For my uses and needs I found SI Pro 2 perfect and very fast on my Athlon 64.

It's all a matter of personal preferences- but I think ANY 3D render that takes more than 24 hours is TOO LONG- I would rather use image interpolation with Photoshop. I personally believe that good commercial art can be made from any decent 3D image as small as 2K x 3K and REZ'ed up using Photoshop interpolation. Especially when you are printing on heavy watercolor paper (like 140# Arches) or Fine Art Canvas...