I'm 6'3" and own a Kifaru Siwash, and have owned a MR Nice frame with both the Crew Cab and 6500 bags. IMO the Nice frame is too friggin' short for larger guys. The 6500 bag has stay extensions help with the lifter strap angle, but the Crew Cab was a huge no-go for me. Ultimately the very flat nature of the NICE frame didn't work with my back curvature either. Others likely have different experiences than mine.

The quality of both packs are top notch. MR packs are impressively over-built and feature more compartmentalization than K-packs. There's a weight penalty for that however.

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