MSM, the 55 doesn't have yellowing like the 50/1.4.
In my experience, the build quality did go down a bit over time... the SMC's have a different design, with rubber and stuff. Not that every lens has little issues - loose trim etc - but seems like the incidence is greater the newer the lens.
The SMC coating
is a genuine advance, advanced for even today. The earlier coated lenses can produce a different feel to photos, sometimes nicer, sometimes not. To be honest I don't often notice any significant diffrence.
I'd posted a photo of an Auto 55 vs SMC 55, let me dig it out again...
The Super is like the Auto, except the aperture ring goes in the normal Tak direction.
Note the difference in reflections on the front glass.