Which one or both?
Since the 18-135mm is f/3.5 @ 18mm, which is a little less than a stop slower than the 16-50mm, it would depend which camera you have. I have the 16-50mm and it's not so good at f/2.8. I usually have to stop it down to f/3.5. I don't know how the 18-135 is wide open, but I don't think it would be worse than the 16-50.
If you have the K-5, you'll just have to push the ISO a little bit higher with the 18-135, which the K-5 can handle quite good up to ISO 3200. The downside is that you'll lose 2mm on the wider angle.
If I were you, I'd just take the 18-135mm. If you have time and money, maybe getting a small bright prime would be better for low-light shooting.
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