For professional work, you should never rely on just GPUs. There’s just way too many problems with them (just take a look at how many GPU related threads there are in this here forum).
You should have a baseline of CPU power that ensures you can get the job done. You should also have a plan to rent out extra CPUs through services like Render.st (or similar), should a tight deadline approach.
There’s several reasons to favor CPUs over GPUs in general, and I have outlined them here. Cycles is not particularly well suited for Archviz, since interior rendering is rather inefficient in it. On the CPU, there’s a much wider array of software available (though you probably will stick with Cycles because of superior integration).
We have really big scenes to render so please take that in mind. I’ve googled about this and it seems that if the scene is too big or complex the GPU may run out of memory.
If your scene doesn’t fit the GPU memory, you can’t render. If there’s some odd driver problem, you can’t render. If there’s a GPU-related bug in Blender that only happens in your scene, you can’t render. Fixing these issues yourself may cost a lot of time or may be impossible.
Granted, any bug in Blender may ruin your day, but a “general” bug (if properly reported) may be fixed more readily than a GPU-related bug (i.e. one that doesn’t show up on the CPU). CPUs are simply much better to debug and develop on.
We thought for the beginning to buy 5-6 E5 2670 processors and to put them in three machine or two GTX 1080 GPUs.
Don’t make it an either-or, get that baseline of CPUs and put good GPUs in your workstations. If it can’t be both, go with the CPUs.
What would be the better option in terms of performance and what do you advise?
You are running a business, you should value stability and reliability much more than performance. Still, if you look at performance in the BI benchmarks, you can observe that CPUs come out on top several times. This isn’t reflected in that BMW benchmark everyone is rendering. Complex shaders, Subsurface Scattering, lots of transparency - all of these have significant performance implications on the GPU. That assumes your GPU can render the scene at all, of course.