arc blast vs. gg gorilla ?

I've owned both, but I'm not sure what my opinion matters because ultimately it depends how they feel to you. If you really like the Gorilla you could just go with that and save yourself the hassle, or you could order an Arc Blast to see if you like it better and return it if not, either way you'll get a pack you will be very happy with.

Personally I like the way the Arc Blast carries the weight much better. And I also prefer any type of curved suspension that allows free airflow across my back, so the Arc Blast has a big leg up for me, but not everyone likes that. The Gorilla (mine was 2013 edition) probably was a tiny bit more cushy-comfortable to me, but I also found the shoulder straps too wide for me and gave me bruises at the ends of my clavicles (those bony bumps out on the end of your shoulders, which BTW shouldn't be an issue with the narrower shoulder straps of the new edition). In the end the Gorilla wasn't the pack for me, but I can see why so many like it.

Obviously the Arc Blast is lighter, more customizable, and more expensive.