I settled on a 50/1.8 AIS (the nicer one, with the front element recessed rather deeply within the barrel). I had bought a 50/2 AI from Grey's of Westminster and immediately afterward my local shop (the deceptively named High Street Radio in Croydon) got in a mint 1.8, and the proprietor kindly loaned it to me for a comparison with the F2 version.
I found the 1.8 slightly but definitely sharper than the F2. It might come down to sample variation--the F2 has a reputation for being quite sharp, and there is some opinion online that it has slightly better bokeh (not something that's terrifically important to me in a fast 50, really). So I swapped the F2 and a bit of cash for the 1.8.
It's killer sharp, if that's what you're looking for.