Originally posted by Doanh I'd recommend the Tammy as it is more versatile
I disagree. The reach of the Pentax makes it much more versatile.
Long story short:
Tamron : cheaper, faster aperture, slower AF, slightly more CA, focuses close, internal zooming
Sigma : faster aperture, faster AF, silent AF, internal zooming
Pentax : better build quality, WR, wider AND longer range, lighter, smaller, slightly better IQ, silent AF
I went for the 60-250 and never looked back. It is as good as the others at f4, the wider FOV at 60mm is really a perk, it's better designed (gotta love that foot). It can be found relatively cheap but never as cheap as the Tamron however.