I have programmed in many languages and find C++ very difficult to figure out and be productive in, compared to other languages that I have used such as Java, C, Python, Fortan, Javascript, ASP and Perl. I would almost say it's in a category of its own. I do NOT believe that competence in one of the languages I just listed (including Java) is a great predictor of how good a C++ programmer a person might be. I think C++ almost requires a different type of thinking, and certainly the ability to hold a huge amount of information in one's head while coding, versus focusing mainly on the business requirements.
I have been trying to become proficient in C++ for years, despite never needing to use it at work (i.e. studying in my own time). It is very tough going and after all this time I am not sure that I would be successful and productive as a C++ programmer. I think I am reasonably competent overall in those other languages.
So, based on my small and clearly biased sample of one person (me!) I would suggest leaning towards developers who have demonstrated a substantial amount of past C++ experience if that is what they are going to be doing at work, day to day. You will need to spend more time finding those candidates but you will have a better chance of success in the long run, I think.
