Sigma 18-50 or Pentax 16-45

I'm about to purchase a replacement for my K10d kit lens.

The choice is the Sigma 18-50 f2.8 or the Pentax 16-45 f4. Putting aside the fact that the Sigma is about 70 quid more than the Pentax, which is the best?

What about the Tamron 17-50 or th Sigma 17-70?

I think you will find most will say the 16-45 which is the one I am gradually leaning towards.

I don't know what a quid is.
around here we trade in Canadian dollars, if we run out of beer.
If you're talking more than a 12 pack, I'd opt for the Pentax 16-45.....
get them while you can, they'll be gone soon.

The pound sign does not work on my keyboard, hence the traditional quid; however if I buy the 16-45 which is a 5yr old lens design, I lose the advantage of the f2.8 the Sigma can offer plus it's a more modern lens design than the Pentax. Decisions, decisions!

Although the Sigma lens is faster and even if it is a newer design that doesn't mean it will be better quality. The 16-45mm is also a modern design, certainly not long in the tooth by any stretch of the imagination.

Generally I think that Pentax lenses are made to higher standards than independent ones, so my choice would be clear. Especially as the Pentax is currently available for a very modest £199 from SRS.