The Boxster S without doubt would be the better of the two. Bigger engine advantage in performance and quickness. I use mine daily for work 10 miles each way and I love it. The brakes of the S are the same as in the 911 there are no better brakes than that. Therefore the equation comes down to price. Which in the use market can be not much difference. Make the investment and you will not kick yourself in the pants later on with the proverbial line I should have.
Gas mileage wise both are pretty similar and in short trips nothing is economical. Over 20 miles to a gallon is normal and in the highway even better.
