Whats your budget?
Do you prefer lux options like the premium package (big screen, leather, cooled and heated seats etc) over performance options like the performance package (3.73 torsen, upgraded suspension , 6 piston brakes, braces and radiator, etc)?
That 17 convertible is optioned with both packages, good color, etc. That's not a cheap car new. I'd try to get it for 30k and think that'd be a good price for that car.
I'm assuming you aren't concerned with outright performance looking at convertibles, so a 17 vs 18 is less of an issue. I think the 15 to 17 look better too.
Performance wise the 18 is a big improvement though. It might be worth spending a bit more for one.