Depending on whether you use the short or long end mostly. If you using the long end mostly than every mm counts, 300mm will not even be enough and adding a 1.4 TC to the Sigma give you a longer reach of 450mm (630mm for APS) at constant F5.6. There is the Bigma but it is slow, so if you shooting motion it will not be good enough. If you shoot on the short end mostly but having a reach of 250mm for occasional use, getting the Pentax is the most ideal.