Originally posted by johnyates The two focal lengths give *very* different results.
What he said.
For the uses you describe, I would suggest the Pentax-A 50/1.7. It is long enough for portraiture, has decent bokeh, is light and compact, and will not break the bank. If you want better build quality and don't mind doing stop-down metering, the Pentax-M 50/1.7 or Pentax-K 55/1.8 have better build.
Either way, with manual focus lenses on your camera, nailing focus will be difficult unless you have an aftermarket focus screen with some sort of focus aid (split image/microprism or fine matte such as the Canon EC). The stock focus screen on the K-5 provides DOF in the viewfinder approximately equivalent to f/4 with the result that missed focus at larger apertures is a virtual guarantee.
Steve