@Gen : Yes I think Arnold but don't count out Vray , is the main cause.
I am not 100% sure but I have a feeling that Arnold was/is gobbling up their marketshare and they have underestimated Vray for the longest of times as well, so let's hope for them that this works and that it is not to late as people tend to finalise their pipelines at a certain point ( and from then it takes along time to convince people to use something different)
I agree Zbrush has a wonky interface, but the tool is briljant though, coupled with how Pixologic do their business ( getting free upgrades and solid upgrades) I think it is the best choice when it comes to sculpting.
Don't know why you don't get on with Arnold as it has a very simple mechanism to tweak renders ( samples and raydepth).
Renderman is very cool as well, just contact your reseller for a free trial ( if your in the UK it's Escape) they will probably oblige and set you up with a licese until Renderman 19 comes out after which you can still use it for free if you don't use it in a commercial way pretty sweet deal!