Ok, first you dont just change tyres based on what you think looks cool. You do it based on what your cars speedo is tuned for. Find out what your stock tyres are and compare it to both those sizes. Choose the size that suits best to keep the speedo accurate.

Use this website -> http://www.1010tires.com/tiresizecalculator.asp

And, to quickly explain tyre numbers. 215/40/17 is width in mm/profile(sidewall is a % of width, so 40 profile means the sidewall is 215x0.4mm tall)/wheel diameter it fits on