Aerodynamics will outweigh weight-savings in all but the most extreme cases -- obviously losing a few spokes and adding four pounds won't make you faster.
Those Nisi rims you talk about are totally box shaped and are single cavity rims to achieve their weight. They won't only be incredibly un-aero with 32 spokes front and rear, but not nearly as strong as a deeper rim -- deeper tends to equate to a stronger rim via vertical stiffness, which is why manufacturers can get away with fewer spokes. Bear in mind that deep rims also have shorter spokes.
If you don't race, then considering lightweight over aerodynamics is stupid. All those non-competitive riders with Ksyrium SSL's should ditch those things and get something seriously aero if they really care about their performance.
Weight starts to matter more than aerodynamics when your speed dips below about 14mph and you're going up at least a 5% grade. At around that speed, the aeroness and lightness will offer equivalent perfomance. If a given race is 80 miles and has four ten mile climbs including a summit finish, and you know you're going to be grovelling at 5mph at the end, then get the lightweights.
I would say that unless you are RACING up climbs that are not only STEEP but also TWO OR MORE miles long, the advantage will always lie with the aero wheels assuming that you're not talking about more than a few hundred grams difference.
BTW: These days, with money, you can have both incredibly light AND incredibly aero. Zipp 404 rims laced to American Classic Hubs will tip the scales at aroung 1150 grams -- and be super aero for those times when you want to go for a 50 mile solo escape.