It doesn't cost any more to buy the right size (unless, it's sale season, but that's another story). You'll look a lot more stylish if your clothes fit well. Find your right size. If you have to cop a slim shirt, buy a slim shirt. It's much better to buy a cheaper shirt that fits great, then buy an expensive shirt that is ill-fitting. Plus, clothes that fit spot-on will take more emphasis off what you're wearing, and bring more attention to and your personality (and that's what matters most).
Focus on Fit, Not Blowing People's Minds