There isn't an ECO model anymore, as all the new gens have the same powertrains and materials.
My LT gets about 46 MPG on strictly highway driving (did a trip counter for 100 highway miles last week actually), otherwise I usually get about 36 MPG in mixed driving.
Folding seats and center arm rest come standard on all models. I have the upgraded radio & bose sound system that actually does a nice job, sunroof, foglights, sport spoiler & heated cloth seats. Should be on par with the old eco trim level, minus leather. Premier basically gets you that and standard leather seats, plus lane assist, collision alerts and nicer rims. Wasn't worth the markup in my opinion and puts you more upside down, as resale value is atrocious on these things. My 11 month old Cruze that was 25800 is now worth about 13K on KBB. Glad it's leased.
RS model is available on LT and Premier.
I also own a 2014 and the new 1.4L in the 2nd gen is night and day. Honestly just about everything is better in the 2nd gen, especially interior materials. It also a much quieter cabin. They even carpeted the wheel wells to prevent noise intrusion. Much more responsive, especially in the lower gears. I notice I can zip into a busy road while waiting to turn much easier, effortless. 0-60 is in the mid 7 range, and passing punch is much, much better... although it falls off after 75MPH. Trade off is direct injection can be a bit noisy when warming up, otherwise it's a much quieter engine. Start stop is love or hate it, still not a fan but I'll deal with it. Make sure all recalls are done on the car before taking the keys (think that's a requirement now a days for new car sales, but it'll save you grief with the center stop light recall). I do think though that the 2014 models are much better geared. Jerky shifts and acceleration are the pitfalls with this car. Never have gotten used to it.