Anything else I might get to build in my life will get done with offset double 2x4 walls. You all but eliminate thermal bridging, get superior sound transmission reduction, and can get very high R value walls using relatively inexpensive blown in cellulose. Material cost is close to 2x6's, a bit more labor, but you make up the cost because you can achieve very good insulation much cheaper-- no exotic materials or need to wrap the exterior in foam to limit thermal bridging. I think it's really a winning strategy.