I will be heading to India for a fairly lengthy trip with lots of photography. My primary camera will be the A7R with a full complement of FE lenses. Since my A7R recently died and had to go to Texas for a repair(very quick turnaround by Sony!), I am particularly sensitive about having a good backup. I will either buy or rent another A7R or an A7S. I would appreciate hearing real world experience as to whether the A7S provides a significant upgrade over the A7R in shooting at higher ISOs. If it's "just" a stop or two, I don't think that's such a big deal, and I am not thrilled about giving up 36MP. (The 4K video is irrelevant to me.) OTOH, being able to shoot clean images at 12,800 ISO versus 800 ISO on the A7R, that is a big deal. That would open up opportunities to shoot at lower light levels at night, the early morning and very late afternoon.
Thanks for any input.