Why polo owners? Even I may help you in this.
See, when you switch to 15" rims, the width of the tyre will increase and sidewall will become narrower.
This has pros as well as cons.
PROS:
+Better handling, cornering, etc
+High speed staibility
+More traction
+Good Looks
CONS:
-Bad on pothole filled/bumpy roads
-May touch the wheel well on bumps/breakers when the polo is in full load.
If the roads in your area are nice mouth-watering and CEMENTED, then go for big rims (15",16").
Otherwise, stay with stock rim size.
Aditya.