Originally posted by lotech reading review I am inclinded to the Tamron due to it's look, size and the AF/MF clutching switch over design
The Tamron is ok, quite good from F4 onward in fact, but that clutch is probably its biggest flaw IMHO. It means you have to make two operations to switch to MF, the gears in the clutch are inaccurate and lead to movement when you engage the clutch, in short it's inelegant.
Having tested both the Pentax DFA and the Tamron for Pentaxforums, I'm able to say that you get what you pay for. The Tamron is pretty good, the Pentax is close to the best lens the company ever produced.
My own choice, to balance reach, size, weight, optical quality, is the Pentax 60-250 F4 modified so it works on Full Frame.