I was in the same situation two years ago and went the full frame route. Two years later, if I had to do it again, without a doubt, I’ll pick up full frame no questions asked. Picked up a 6D actually.
Low light and the fact you want to shoot wide are reason enough to go with full frame over a crop sensor. Add to that Canon’s large lens selection, and it’s pretty much unbeatable.
About the 6D, it’s a very good camera. People say it’s a "budget" full frame camera and in many regards, they are right. And it’s up to you to judge whether you are ready to give up some stuff for going with a 6D (below is are my top picks which I would’ve hoped were better) :
1. Nowadays, the 6D’s sensor is quite limited. It only has 20 Mpx and a very narrow dynamic range (I think about 12 stops). However, its noise performance is solid and surpasses the more expensive 5D mk3.
2. The 6D’s autofocus is also limited. At only 11 points with only one of them being cross-type, it’s as entry level as one can get. However, again (the 6D is interesting beast), its center point is quite the performer and can focus at -3EV. The more expensive sister (5D3) can’t do that.
3. Only one single storage slot, SD. Which can be quite unacceptable for people who want redundancy.

The 6D also does GPS and WIFI. But quite honestly, they are so badly implemented and I have never used them.
The three drawbacks I enumerated above are annoying but don’t take away from the 6D’s appeal. For example, I am quite content right now with the sensor’s abilities (and am actually very impressed with the noise performance), I don’t mind focusing and recomposing, and I wouldn’t care if I lost a couple shots once in a blue moon (not a professional, I do this for me).

Have you looked into Nikon yet? Their competing D600 series is quite good and overcomes many of these drawbacks. Although it comes down to personal preference. While Canon have shit sensors, their lenses are at the very top of the food chain.
Now if anybody could preach for the Fuji in order to have a full picture and make up your mind, that would be swell!