I understand your view about sticking with the K5 and investing in better glass. If you want very good glass then don't go down the sigma zoom line. The sigma zoom is OK but not brilliant. It comes down to how much you want out of a lens. I like kicking butt. I used a 150-500 sigma in Africa and after the event was really disappointed with the images compared with my Sigma 500 prime.
Both pentax lenses are excellent choices. The pentax 1.4 on my 60-250 zoom sucks. I don't have or need the pentax 300 so while I have no experience of the combo, others give it rave reviews.
Hope you really enjoy which ever choice you go with. New glass after you have been starved for a long time is very exciting.
Last edited by Bob from Aus; 08-02-2015 at 06:44 PM.