I've used GF for many years -- basically since it was first introduced, and used it with my scanned images, and later some of my digital images. I tried Blow Up when it was first introduced because Alien Skin has always produced excellent pluggins.
By my comparisons, there weren't many differences and a lot of comparable qualities. The slight differences were Blow Up's ability to add faux grain, if you feel you need it. Personally, I don't. The BIG plus is if you work in layers and want to res-up a layered file. Blow Up allows you to maintain the layers, GF makes you flatten the file. Speed-wise, I don't remember a dramatic difference between the two. Both were pretty slow on 16-bit files, Blow Up was a little faster (if I remember correctly) in 8-bit (without layers). Overall, there wasn't enough of a difference for me to buy Blow Up. Thinking about now, perhaps I'll give it another try. (I also own pxl Smart Scale, so I really don't NEED yet another program, though pxl Smart Scale has a better, full-screen preview, but runs out of memory easily.)
Overall, I don't think you'd go wrong with either. If you're only resizing a little, use PS Bicubic smoother. I use it more these days, because it's far faster, and quality is much better than in previous years.