The series of the tire is a percentage of the width. A 205/40 tire has a thinner sidewall than a 215/40 tire.

That said, I don't believe that a 17" wheel will provide better handling than a 16" tire. The slightly thinner sidewall afforded by running a 215/40 tire as opposed to 215/45 is not much. 10.75 mm to be exact. The stiffer sidewall is offset for the most part by the higher unsprung weight of the larger-diameter wheel and tire. Weight does not only affect the acceleration.

Basically you will have slightly better initial turn-in with the 40-series tire but that's about it. Plus you have to spend more money to get the weight the same as a comparable 16" tire. Aesthetics are a personal choice, and I'm partial to 16" wheels as opposed to 17".

I think the benefits of lighter weight and lower cost outweigh any perceived aesthetic benefit of the 17" wheels, especially since there is no real performance benefit.