I would consider the Sigma 70-300mm , nice lens and much better macro ability than the Tameron 75-300mm (1:2 vs nearly 1:4)
Most who have them seem quite happy with them too. I am.
Some nice examples here
As for the two prices on the Pentax, probably the more expensive one is an APO version.
There are two version of the Sigma too... regular about $129 aand the APO version about $200.
I have never heard much on the Pentax 75-300mm, so can't really say much about it.