There have been several good points brought up here. I have to throw in my two cents as well . . .
I don’t have any experience with Arion, so I honestly can’t compare any of these to that . . . however, I think your choice ultimately depends on what the bulk of your daily work renderings will be.
I’ve got seats of Maxwell, Octane, Vray, and Thea and have played with Enscape. My thoughts, in a nutshell are:
Maxwell : quality is top notch, but for what I do, it’s just two slow. Even with the GPU support they’ve added, which has cut several features out, btw.
Octane: this is what I do the bulk of my work in. 90% of my renderings are exterior architectural shots or products. I do several product animations each year. I could go into great detail about this engine and won’t unless you want me to, but I do want to at least point out, that it’s extremely capable of some sophisticated materials and render setups. I can get faster product renders and animations out of this engine than any of the others on this list, and with quality on par with each of the others, so there’s no quality sacrifice to the speed. So . . . i think if the bulk of your work is product based, I’d urge you to really check out this plugin in. The Rhino Plugin Developer, Paul, is very responsive as well and really listens to the Rhino user feedback.
Vray: if the bulk of your work is on interiors, i’d definitely lean towards Vray. You’ll get high quality and faster interiors with this engine than the others in this list. The UI for the VFR plugin is the best as well out of all of these on this list. I do have some development concerns with updates and such with VFR based on the past, but maybe that is going away. But still . . . an awesome render engine.
Thea Render: Thea attempts to do it all, and it’s a great engine. However, it’s user base, at least for Rhino seems small, and it’s development has been slower than I’d like. I’ve pretty much stopped using Thea however, because of it’s lack of animation support with Bongo. That’s a deal killer for me.
Anyway, personally, I’d download copies of both Vray and Octane and spend some time with them and see what works best for you. I don’t think you could go wrong with either. My choice to do most of my work in Octane was simply based on the fact that for my daily work it was more productive for me than Vray. Same quality and faster results for all the product shots and animations.
Hope this helps in some way,