It all depends on your needs. If you're okay with SU's toolset and poly limitations, go with Vray. No need of going through the hassle of exporting to modo, cleaning up your model etc then.Then again, if you think you'll be needing a bit more in the future, get Modo. If you buy 301 now, you get a free upgrade to 401.
personal observation Me, I got both. Yet, if I were in your shoes, I'd choose Modo, as it doesn't look SU is going anywhere, for the moment.