GPU Replacement Slot A (Compute) or Slot B (Render)? Lines on Display

Hi,

I have a Mac Pro A1481 2013 where one out of the two AMD Fire Pro D300 GPUs has failed. I'm looking to source a replacement but am unsure which to replace.

The machine powers on but shows lines on the display (see image attachments below).

Any suggestions / advice would be greatly appreciated.

Ben.