Going 205, will alter the cars gearing slightly and in turn modify the speedo readings. But should be very mininmal.
Not too sure which way the gearing will go, think it will increase your overall wheel diameter (as the 45 profile is a %age of the width i believe!) so you will have taller gearing (higher top speed) but isnt a huge change.
Id imaine grip would be slightly improved.
Final not, more overall diameter, more weight! (I guess)
Hope that helps.
Im guessing the 195s will be cheaper, as more common.