rsimoes,
The question plagues us all. You will see that the topic resurfaces every few days. The crux boils down to authentic "look" v availability of performance tires for 15" rims. I like the look of the 15" rims but there is only one true performance tire (Goodyear Eagle) in the correct size. The link below is a place to start.
17's on FIA
Good luck, Pete C