Hi,
I don't have the Sigma but the HD DA 20-40 limited which is optically better I think?
It is a f2.8 lens at 20, f4 at 40
It you are interested it is on sale on the forum here :
https://www.pentaxforums.com/forums/24-photographic-equipment-sale/302261-sal...mited-box.html
It is still under warranty
Regards