After handling both in person, I think the Ohm wins in a landslide. The Ohm 2.0 is larger than the Gorilla, and I didn't care for the Gorilla's boxier construction. Obviously, a lot of people on here like the Gorillas--and personally, I think their hoop stay is brilliant--, but I think the Ohm 2.0 has a much stronger hip belt attachment and the overall pack design is stronger (excellent side pockets, properly angled shoulder straps, better compression). The only place the Gorilla probably beats the Ohm is in materials. But Ohm users report excellent durability of the 70d fabric.
If you really like the Gossamer Gear packs, the Mariposa is probably a closer comparison in size.
Obviously, fit trumps everything. It's worth the extra shipping to get them both and load them up and try them on.
/*/Edited for spelling./*/