With utilities going to time of use power pricing programs, the gas engine drive may be worth a 2nd look. Prices during peak hours in our area are much more expensive than off peak. By getting the "big boy" off the grid during peak times it has a HUGE advantage in utility costs. In our area NG goes for 40 cents per therm which is the BTU equal to 2 cents per KWH. On peak rates are typically 22 cents per KWH, off peak is 5 cents per KWH. The gas company makes their money on the $27 monthly meter charge, while selling the gas relatively cheap.
The details are what kills you. Homeowners don't want to mess with their A/C unit. It hard enough to get people to change their filter and clean their condenser, much change oil, tension belts, etc. They want to turn it on and just be cool in their house. I can more see the gas engine units being used more in commercial where they have somebody to maintain them and the savings would be greater due to the larger unit size.