1. They all have very similar features, but the Corsair RM850i has corsair link, which some people find helpful.
2. I would say that they have equal customer support from my experiences with both companies.
3. If I wanted the highest efficiency, I would go with the Corsair HX850, as its 80+ platinum. If I just wanted a PSU, I'd go with whatever was the cheapest corsair option which can power everything I need, which in this case, would be the RM850X, which as a bonus also has braided cables.