The 870 is 4K and the 770 is HD, so what are you doing with the resulting video? They both have the same size sensor chip. The 770 should be cheaper so may be better value for what you want.
The bigger the sensor and the lens, the better for low light but I expect both will rely on their video amplifiers to boost the picture. (the 870 has infra red mode!!)
On balance I would go for the 770 as the 4K addition of the 870 may be of little use for your needs.
I'm tasked with getting a new camcorder to record church sermons. The video will be taken from about 30' back in a dim lit room, with spotlights on the stage. The budget is $800. I've narrowed down to Panasonic HC-VX870 vs. Panasonic HC-V770. Based on this the V770 seems to be better? But I don't really know if sensor size is more important than illumination and zoom?