20-40 or 18-35

i have been looking at getting a replacement for my sigma 18-50 2.8 that i got when i bought my K20 about 9 years ago, i aquired a K3II a few months ago and was thinking of a newer/better zoom, i was looking at a pentax 20-40 but when reading reviews the sigma 18-35 1.8 was mentioned a few times, so i read some reviews on it as well and it seems to get a good write up, i know the sigma is larger and heavier as it is a 1.8, has anyone had both or experiance of both any advice on the pros and cons of both would be welcome.

I have both. They are both good quality lenses. Many people have experienced focus issues with the Sigma, particularly when using multi-point AF. I generally use centre point AF and have not had any problems. Other than that the main difference is size and weight. The Sigma is huge next to the Pentax and weighs 3 times as much, so if that is more important to you than maximum aperture, go for the Pentax.

The obvious choice would be the newer Sigma model, which is 17-50mm.

I remember buying an 18-50mm as my first "good" lens for my K200D and took it back the next day as it was very soft wide open. I changed for a Tamron which was much better optically, though with poorer build quality.

More recently I got the newer Sigma and sold the Tamron as I like the Sigma more. Optically they're on a par but the Sigma is much better built and has an in-lens focus motor. It's also significantly heavier but that's the price you pay for good build quality.

Depends on the type of shooting you want to do, Sigma 18-35mn is f1.8, which makes it pretty nifty, indoors, as it's pretty much sharp from wide open. And you can get some decent close up shots at f1.8 and experiment.
20-40mm good walk about, small filter size, 55mm as far as I can remember, way smaller , lighter than the 18-35mm.
I have had both, but if I was looking for coverage in those lengths I'd choose neither, and get a pentax 16-85mm instead.
Just my tuppence worth.

Derek

The problem with the 16-85mm is that it's slow, which makes it much less useful in low light. A very different lens from an 18-50mm f/2.8.

Ah I know that, which is why I said it depends on the type of shooting he wants to do.

Thanks for the replies, I was thinking of a lens with better IQ most of my shots are taken between 20-30mm I have other lenses covering 10-24, 18-200, 70-200 2.8 and a couple of older primes and zooms from film days, I read that these two lenses give prime quality images, the sigma appeals to me because its 1.8, size and weight don't bother me, I have a Pentax 50 1.7 and a 28 3.5 but haven't had great success with them either on the K20 or K3ii maybe something I'm not doing right.

Is the 28mm f3.5 you have, the K 28mm.
That's really well regarded and when I had one, i just loved the iq.
I have no experience with the newer sigma that Jonathan-Mac suggested. But to be honest, either of the 2 lenses you initially suggested should be capable of giving you the results you want. It's a toughy

Derek