To help you in your choice, you can consult this section:
There is a link to a Google spreadsheet where the performance of GPUs (and others, like CPUs, DPL versions 4 and 5, and so on) are compared on test images.
This topic, and the corresponding spreadsheet are very informative.
For example, switching from DPL4 to DPL5 and updating the GPU driver is probably the most efficient way to reduce DeepPRIME’s processing time at low cost. You can also guess the expected processing time for DeepPRIME based on the FP32 score of your GPU.
Some results for DPL4:
And for DPL 5:
Regards,