I’m friends with the (now former) owner of Giant Cycling World here in Denver and they haven’t seen any major issues. The shop by my house is a Niner dealer and they speak equally highly of that brand.
The geometry is pretty different between the Giant and the Niner, so it would be a good idea to grab a test ride on each.
I wish we had more dealers around. It would be virtually impossible to ride both from the same shop.
The Niner is aluminum, has better components and is a few hundred cheaper.
The Giant is Carbon with a pretty nice set of components for the $$
coming from a 26" Rockhopper – they all feel different to me!! This is a tough one.