195/50/15s will have a sidewall of 97.5mm
205/50/15s will have a sidewall of 102.5mm
5mm differance.
205s will have 10mm more width of tyre in contact with the road, so better traction and grip.
195s will be more economical on fuel - less contact with the road.
Depends what you want the tyres for.
Cheers
Paul