The 3.6L is a significantly better engine overall and the fuel mileage bump is also pleasant difference between the two, though only a few L/100km. That said, remember you're all driving on winter fuel right now and that hurts it quite a bit as well. Wait until the spring time, break in the engine a bit, and then you'll have closer to real numbers.

 

My 2012 JKU automatic is getting 14.3 per 100 after one month. 99% city driving. First oil change will be putting in a KN airfilter, really helped on my Mustang so keeping my fingers crossed that and break in will improve the gas mileage on the Jeep. After the first week on checking the read out I was freaked, it read 26.9 per 100. I forgot to reset !!!!!!