Same problem here. It is hard to recommend. I am still waiting ATI will kick nVidia ass in driver and software support, whether it will be from Opensource or fglrx side. But that day seem to be far away.
Good thing is, Ubuntu has some premium support from ATI, which annoys users of all other distros. I don't understand this selective approach, but it is good for you. In a GPGPU world, there is nothing comparable to CUDA, so it is hard to guess if anytime and when OpenCL will arise to the same level of support, productivity and popularity. If you won't dualboot Windows, I would rather go nVidia way, because it works today (of course support OpenCL), and it is sure thing in workstation market.
But this could change. Even today, you can see OSS drivers has better 2D performance than binary blobs (which share the codebase with windows drivers). As i said. It is hard to guess.