Which external renderer should I use for architectural visualization? I use Blender mainly for creating architectural renders. I love Cycles, it's nodes, quality, realism, everything about it, except for it's speed and noise.

An indoor night-time scene with a lot of small light-sources seemed almost impossible, somehow I've managed to do it but I had to pull all kinds of node-tricks to get a relative low-noise render and still lasted 5 hours per image (1800x1200) on a mid-high range GPU to get acceptable results.

Is there an alternative render engine for arch-viz? It doesn't have to be completely free, but has to work on GPU, should be well-integrated into Blender, node-based engines are preferred. Cycles would be perfect without the noise.

Thanks for any input, and sorry for asking but I have no time to try all the existing external renderers, since there are so many of them now.

Cheers!
You would choose a raytracer that has features that are required for architecture projects. 

An example is YafaRay. YafaRay supports features like Global Illumination and Background Illumination which are specifically useful for architecture visualization.

Cycles has Global Illumination (since a few years ago) and I am not sure that it has as good Background Illumination as YafaRay. 

YafaRay is distributed with an open license and you can use for free. It is designed to be used with Blender.