Most important thing is what app you work with and what monitors you hook them up to.
If you use DP or HDMI 1.3 enabled monitors or multi-monitor configuration it matters a bit, if you use apps that prefer one or the other architecture it matters, but a majority of the apps out there do their acceleration via OpenGL and it's available to both in one way or another.
Main ting to look for is good quality mid-range with low power requirements. I wouldn't bother with an HD4850, too much power consumption, a better choice in that space would be the HD5750 or HD5670.
My personal preference would be for an HD5670 which gives you a balance of performance and power consumption as well as new features.