I'm stuck between the 50mm 1.8 Series E. 50mm f/2 AIS. and the 50mm 1.8D
There wasn't a 50mm f/2 AIS, only AI were made. I have an early AI version and it is excellent.

The series E and the latest AiS 1.8/50 without coupling has only a minimum distance of 0.6 m and has a simplified coating.
As a curiosity, I have a 50mm f/1.8 AIS with rubber focusing ring and a minimum distance of 0.45m and no "rabbit ears". It is part of a series only sold in Japan, but I bought it second hand in the UK!