The main Benefit of having a T56 is it can take more power... Stock its suppose to beable to take 750 hp.... The stock nv3500 is only good up to 280 hp i believe... So even if you upgrade the clutch or something on the NV3500, there's still parts in there that will break after enough high power abuse
(disregarding the freak transmissions that will take more power, and the clutch too.... Like some of those trucks, where you can put too of the same stock trucks next to each other, and one will be faster than the other)
.... The T56 on the other hand will be good..... For me too i'd like to eventually put a 6.0 in my truck, or maybe a duramax, and i'll won't have to worry about my tranny at all.