Do the 2x4 on 16" centers, screw the insulation. Unless you heat the building 24/7 you will probably never save on fuel what you will spend on the incremental increases in the lumber, insulation and extended jambs. Everyone gets a raging hard-on over mega insulating the **** out of everything without regard to what it's going to end up costing them, or maybe put differently...what it's not going to save them. Think of your project as a big pie... the pie is the cost of the buiding and what it costs you annualy to do the stuff you want in the building. You can decide how you want to slice up the pie i.e. heavily loaded up front with an expensive construction cost and a lower annual cost or saving the money up front and spend it incrementally in your annnual costs. Throw in a dose of reality along the lines of "What if I don't end up staying here for a long time?"...What if my interests change and I don't use the place at all?" What if I decide to sell it and no one gives a rats *** about how wonderful my garage is and I never get back what I put into it?"
Out of any of the things you could do to build an energey efficient (but not necessarilly cost efficient) building, the wall insulation is actually the smallest part of the equation compared to good ceiling insulation and infiltration management. The other thing you gotta remember is once you open the garage door, It doesn't make any difference if you're in a R90 thermal vault or a freakin blue tarp shelter... you have pissed all your heat outside anyway.
