I personally think Thea is better than Octane, but feature by feature it may be a draw.
I have Octane for Lightwave and while it's well integrated, I hardly use it.
Octane 3 has hair, volumes and some other stuff like that, which Thea hasn't (yet).
Thea has GPU & CPU at the same time for quite a while already and it really flies while Octane only announced it so far AFAIK.
I like the material system in Thea better and while I normally am a total node-addict, I find the nodes in Octane rather limited and clumsy.
Thea feels more versatile to me and has a clearer workflow IMO.
ATM, a network rendernode for Thea is 49.- Euro and two are included in the initial purchase - Octane is much more expensive there.
Theas network setup is the most brilliant and fast I've ever encountered in 20 years of CG. Once set up (5 minutes per node max) you simple enable network rendering in Thea and all machines contribute very efficiently.
But if you have Octane already and like it, it may not be as interesting as if you would start from scratch.

Cheers,

Tom