NICE: More weight. Greater lateral / torsional stiffness. Thicker padding in most areas. Extremely comfortable.
Guide Light: Less weight. Greater lateral / torsional flex. Less padding in most areas. Extremely comfortable.
I've owned both. The GL will do everything I want and I found it to have no deficiencies compared to my NICE frame. I believe the GL is a better 'carry' as a day/hunting pack. The NICE might have an edge when it comes to packing huge loads over long hauls. Both are extremely high quality setups. In a head-to-head I would choose the GL unit. If I had neither and someone offered me a deal on a NICE frame I would take it knowing I still have an extremely stout, hunter friendly setup.