Hi,
As above, it depends on the format but also how wide you want to be.
I shoot full frame and I have the tamron 17-35 and did have the 19-35.
I found both to be perfectly acceptable for landscape, they're not ultra-sharp (but by no means soft) but they don't need to be at all, especially as they can be picked up for around £150 and £100 respectively.
If you have a crop camera then there is always the canon / sigma tamron / tokina 10ish - 20ish ranges