Assuming the drivetrain can handle the torque and hp, or requires little modification, I would go with the 2.0L turbocharged Ecotech. In my HHR-SS with 35,000 miles, it has been flawless. The two minor downsides, the direct injection fuel pump is loud (OK here, with open cockpit) and the dogs bark at the turbo whine (OK again if you want even more attention). Seriously, at approx. 785 kg curb weight, the 260 hp Ecotech would probably push (not pull) you as quickly as you want to go, assuming a very sticky drive tire, and my engineering guess on the gas mileage would be in the lower 40s with driver and passenger adding 160 kg or so ... not bad performance and economy.