its better to use 14" rimz.
15" rimz wud cost u more firstly,they wud b heavy so reduce fuel economy,steering wud b harder to turn,ur suspension wud hav bad affects as the air cusion wud b less nd solid alloy wud take its place nd the ride wud become bit bumpy...but on the other hand,it wud look good,wud provide u wid greater grip along corners,stability etc.
so its upto u now.