Here’s some interesting info I found about the GPUs in these Mac Pros that may help with purchasing decisions.
This post has the clocks and TDP info:
https://forums.macrumors.com/threads/nmp-firepro-dxxx-clocks-and-tdp.1688128/
The D300 operates at a higher frequency than the D500. In fact, it has the highest base clock out of all 3. So it would be faster than the D500 for tasks that aren't heavily double-precision reliant. They must have been able to clock it higher and give it a slightly higher TDP because it generates less heat. According to
this post the D300 stays below 70 °C during heavy use. While the D500 & D700 approach 82 °C during heavy use according to
this thread. That's consistent with
Snow Tiger's statement that "the D500 and D700 cards suffer from thermal fatigue issues due to the poor cooling the small chassis provides." Perhaps the D300 doesn't get so hot because the Pitcairn die is much smaller. It's
212 mm² while the D500 & D700's Tahiti die is
352 mm².
There are also reports that the D500 & D700 sometimes have issues with 4K displays, while the D300 does not, according to
this thread and
this post. From my research so far it appears the issues some people have experienced with the D300 seems to be limited to random freezes. I haven't yet read of an instance of a total failure. According to Apple's technician guide for the Mac Pro (Late 2013), "The latest versions of macOS have addressed many issues with graphics issues on this model." So presumably, many issues have been fixed via firmware & software updates.
Based on the info I've been able to put together so far it appears it's most likely that a Mac Pro with any of these 3 GPUs won't have problems, but if you manage to get a D300 system that doesn't have glitches I think it has the potential to be the most trouble-free with greatest longevity.