Hi i have a Sigma 24mm 2.8 Super wide II in Pentax Af mount, it is a good lens sharp, at all Stops, good contrast well built, The other 24's I have are a Tamron Adaptall II which is a very sharp under rated lens, and a Nikon 24mm 2.8 AIS this is a very nice lens, and it performs very well and the most expensive of the 3
I have found the Tamron to be the pick of the bunch, they can be had for around $60.00 with a mount and old tamron lenses are very well built. The only issue is it uses 55mm filters, the sigma and the nikon use 52mm filters
All three lenses are good choices and you will be happy with any of them.
Johnkpap