CS100ER: 133 Seats x 514 Speed / 11,960 Fuel = Efficiency 5.72, Turn 25, Range 3,050
E-195AR: 124 Seats x 553 Speed / 14,060 Fuel = Efficiency 4.87, Turn 30, Range 2,675
737-600: 132 Seats x 526 Speed / 13,650 Fuel = Efficiency 5.09, Turn 30, Range 3,569
The E-195AR is worse in every metric. The CS100ER is most efficient and turns the quickest
The 737-600 seems pointless because with the way AE works, you could just get the CS100ER for your regional network and then use the 737 delivery slot for a larger, more efficient 737 variant. If this is in 2014 when the CS100ER is available, then the 2015 release of the NEO series seems to work better for longer range or higher capacity operations with the -319/-320/-321 depending on the scale of your operations. In real world terms, it would also look like a realistic Airbus order to have CS100ER and A320NEO or A321NEO.
For an earlier 737 variant that would work well, the 737-900ER is best to provide a significant range boost and high capacity and configuration options. It also has a stellar efficiency of greater than 5.81 (I say greater because the mandatory winglets increases this further but the stats are not available for these on the airplane's page). The winglet efficiency boost probably pushes it above 5.9.