Pentax 50mm f/1.4 - manual vs auto focus?

I've had the SMC Pentax-M 50mm f1.4 lens for a while now, and love it. I've taken some great shots with it, and just love the overall image and build quality.

Here's one of my favourites:

However, it must be said that manual focus is not always so convenient - if you've got the time, then you get great results. If your subject keeps moving around, it's a bit harder!

I now have the opportunity to get the SMC FA 50mm f/1.4 at a very reasonable price from a work colleague, but it will require using some of the money I've been saving for a 35mm f2.8 Ltd. So my question is this: is the autofocus the only difference, or will I also see an increase in IQ between the two lenses (bearing in mind that I find the IQ - particularly the sharpness - to be pretty outstanding on the MF version. But then again I'm not really an expert.)

Any advice/thoughts would be much appreciated!