Check out mattdm's site here. He is a Pentax user on this forum and is a great site/write up on all the flashes for Pentax
Pentax P-TTL Flash Comparison: The Definitive Guide
I don't have the Pentax flash but I have the Sigma 530 super. I think the only difference is the Pentax 530 has a little bit more power over the Sigma. Unless if you are shooting at full power all the time(I don't see a reason why anyone would) the Sigma would be good enough. It is a lot cheaper too and does everything the Pentax 530 does. The Sigma works great in Auto mode and I can't find anything to complain about it when it is on auto.
If you don't need wireless p-ttl, or swivel head, the non super can be bought for even cheaper. The swivel head is handy though and I would recommended the super version.