all a matter of taste. I like 17's because they ride better and still handle pretty well, but 18's will give you better handling (assuming tire similarity) but ride a little rougher due to the smaller sidewall on the tire. Biggest benefit to ECO wheels is the loss in weight being forged wheels over standard casts.
for all those who have contemplated the wheel change on the eco. i have changed from the 17" to the very heavy 18" ltz wheel. they fit fine and the mileage difference was not noticeable. i still average 29mpg in the city and i've had the wheels for 6 weeks now. only had the car for 12. just a little fyi