I’m trying to figure out what version of vray I should buy for Rhino 5. I’ll mostly be doing product renders. Any advice is appreciated.

Are there different versions available? I thought Chaosgroup sells only the latest version.

As I understand things there’s Vray Next and Vray 3.

At the Chaosgroup shop I have seen one VfR only, so maybe you get both versions.

VfR Next got a lot of improvements and the GPU support is enhanced. I would go with Next. But I must say I never used 3.6.

Is Next stable and intuitive?

I don’t know how stable 3.6 was. There are a few known issues at Next, maybe also some unknown, but I think, it’s on a good way. I hope more people will use it and help to find the last bugs - it’s an investment in the future.
I think it could be called intuitive. Best you try it. I use it for my projects since a few weeks and it works.
Also Next better supports GPU rendering than 3.6.