Ask Slashdot: GPU of Choice For OpenCL On Linux?

Bram Stolk writes So, I am running GNU/Linux on a modern Haswell CPU, with an old Radeon HD5xxx from 2009. I'm pretty happy with the open source Gallium driver for 3D acceleration. But now I want to do some GPGPU development using OpenCL on this box, and the old GPU will no longer cut it. What do my fellow technophiles from Slashdot recommend as a replacement GPU? Go NVIDIA, go AMD, or just use the integrated Intel GPU instead? Bonus points for open sourced solutions. Performance not really important, but OpenCL driver maturity is.