Basically the Ultimaker philosophy is to create a machine that lets people print beautiful objects - that is where it excels.
While other printers are promoted with great features, Ultimaker gets the basis right, they are customer-friendly and stable machines with the best surface quality.
The Ultimaker 3 creates prints the most smooth surface without any zebra stripe effect created by minor inaccuracies in the mechanical assembly. This is the only consumer-level machine if you want museum quality objects. And the dual extruder is flawless. Now apart from that it is basically the same machine as the other Ultimakers, which to me does not explain the price tag.

I have seen the Sigma and it looks like a great machine, however people weren't able to show me any prints leading me to think that it is not so straightforward to set up.
I am now in between the UM2 or BCN3D+ for a next printer buy.