Very nice bike. The front derailleur I would replace with XT same as rear as well as upgrade Avid 3 to Elixir. Would have preferred a Fox Float RP something for rear shock...
I know Silverback has their detractors, but you have to decide for yourself.
We can only provide opinion. I'd buy the Silverback although I have been looking around at the Specialized...
I'd say try out both. That is the only way you'll know what works for you. GT has a good rep, Silverback is new but may be just what your looking for...