The main difference between S3 and S5 is the powertrain. The S3 comes with a m2DICR 4 cylinder 2.5L turbocharged direct injection common rail diesel engine, while the S5 comes with an mHawk 4-cylinder engine with variable geometry turbocharger with intercooler, 4 valves per cylinder and common rail direct injection technology. 

Feature-wise, the S3 and S5 both come with follow-me-home headlamps, bottle and cup holders, one-touch lane indicator, and fabric seats. Safety-wise, the S3 and S5 both come with ABS, seat belt reminder, and a few other features.