I own both programs. They are basically equal in abilities. I regularly use the to enlarge a clients images for 8 foot tall instore posters. On average, these are 1Ds3 images enlarged to 200% or more. (The printer takes it the rest of the way.) I did however, do an image years ago with the original 1D (4MP), with spectacular results. The client chose a shot for a poster that was originally going to be used for the web!
I find the edges and detail crisper than bicubic. The easiest thing is download the demo of both, take an image and enlarge it with all three (bicubic, GF and BlowUp), crop a section to fit on 8.5x11, and print each, looking at them at arms length and 5-feet. The other thing to keep in mind is view the enlarged image, onscreen, at only 50%. I find that gives a much more realistic view of what the final image (sharpening and all) looks like.
Nemo