I have owned the Sigma 120-400 for a couple of years now. It is a lens that needs to be stopped down to get the best out of it but it does give very good results. That being said the below shot is something I could have achieved with the DA 55-300 since both lenses need to be used at around f8 to get the best out of them. (It must be noted the Sigma IQ tends to fall off towards 400 but is still usable)
The Pentax DA* 60-250 has excellent IQ throughout the range and wide open has superior IQ to that of the Sigma. It is a AW (all weather) lens which is a advantage over the Sigma. Focus speeds for the DA* 60 -250 and Sigma 120 - 400 are comparable.
As Jatrax accuratley states, you are really comparing "Apples with Oranges" when comparing the DA* 60 - 250 with the Sigma 120 - 400 (and DA 55-300 for that matter). It would be better to compare the DA* 60 -250 with the Sigma APO 70-200mm F2.8 EX DG OS HSM which I also have. In that comparison it is a bit simpler - the Sigma has faster focusing, larger aperture
but is a shorter lens both ends and is not an AW lens.
So what is the most important factor for
your photography?