People generally think that Pentax is inferior to Canon and Nikon. I use both Nikon D300 and Pentax K20D. The only real advantage of the D300 is the focus speed. Metering of Nikon may be somewhat more reliable, but not so important. However, despite many direct buttons for control, the control of D300 seem less customizable for user preference than the K20D. I prefer the color, white balance of Pentax. And many other things that make the D300 unable to be a winner of the much less expensive K20D.

I am not totally satisfied with my 16-50 in terms of optical quality and sluggish focus, but don't think I will switch to a Tamron, which arguably sharper. Just because it is a DA*, the pride of Pentax owners, which differentiate myself from a Nikon/Canon + Tamron

2-3 years ago, I can get the 16-50 at very good price, less than 600 US$ I remember.

Just to share with you: My K20D sometimes cannot lock focus with the DA* 16-50, though it is very rare (just about 1% of shooting). Most of the time were focusing at infinity. And I remember one time I could not set the aperture (but I don't remember what lens I was using). But then everything turned back to normal. As the problem is very rare, I don't intend to send it to maintenance service.

I think I will try adjusting the focus correction to see if the problem persists.

Don't sell the prime, optical quality is excellent, great value of being a "limited" lens, and taking photos with a prime is so enjoyable. You will not have the same feeling when using a zoom lens.
For convenience and everyday shoot, just add a zoom lens to your collection: 18-55, 16-45, 17-70, 18-135 WR, or 16-50, depending on your budget