It's funny because I'm trying to decide this same thing. But I say khaki because you can wear it every season. The denim is more for summer. And I think it probably WILL rub off on your clothes because of how close it is because my jeans rubbed off on my dark brown swing pack and I don't think you want that either way around. HTH!
Khaki usually works well with everything. I am not really a fan of denim, but that bag works well with the denim and white trim. So I vote denim. It'll stand out much more and is different. Khaki is too common.