Go with the pre-cast concrete coping in an off-white. We went with a tanish rock salt finish concrete deck with the bullnose coping and I am very happy with the results. It looks 1st class.

We also live in Houston and had our pool built about 2 years ago. We used slate on the coping but regular tile on th water line. We haven't had any trouble at all. But there is some more to the story. My builder has had trouble with slate in the past and really didn't recommend it but my wife LOVED it. He agreed to talk to the local stone providers about it (forget their name at the moment...) and they said that during the time my builder had his issues, there were a lot of complaints about slate coming from particular areas of India. They checked records and found that the troubles were with the same slate my builder had used in earlier jobs. They said that the slate they had in stock had not seen the same issues, so we went ahead with it. We did seal it however with a pretty pricey sealant. We've had some VERY minor flakes come off the surface (not the bullnose end though) which is typical of slate, but overall they look brand new, and we do have a salt system. I do keep my chemicals in check fairly religiously as well. So, not very helpful, but thought I'd mention it...

I believe (based on my own experience, not expertise) that it's just going to disappear soon enough. However, I just bought myself a steam cleaner which arrived today, and I have been happily cleaning just about anything I can find with it!! And I think I would try the steam cleaner on your concrete stains!

I think it looks good. What did you decide to do? Can you share a picture of the final project? Do you remember the color of your pavers?