they'll both put your speedo out to some degree just the 50 aspect ratio will do more so than an aspect ratio of 45.
using 155/70 R13 std
155/70 R13 60mph
195/45 R15 61mph
195/50 R15 63.2mph
i wouldn't choose by that though. My desicion would be made by the range of ultra high performance tyres avail in both sizes and their prices. In australia its very hard to find any 195/45 R15 and the ones avail are not name brands. Whereas there are allot of 195/50 R15 tyres avail in various proven brands and are often better priced than the 45's, all depends on what is avail local too you. I'd try to avoid the cheapest tyre avail, theres probably a good reason its soo cheap, either a its junk or b its a quality tyre been marked down. Prob with option b is it could be very old stock and likey the tyres will split/have problems sooner than anticipated,
Alex