The EQ6 can carry more, will probably track better (belts), have less backlash and will allow better upgrades to your OTA and equipment in the future.
The HEQ5Pro can be a good performer, but older gear driven mounts were prone to significant variability between individual mounts (luck of the draw). My HEQ5Pro with gears still blows away my belt modded EQ6 for tracking (RA periodic error variation), however this is probably a statistical oddity. Don't plan on putting more than 10kg loads on the HEQ5 for astrophotography.
There is the issue of how long you'll be active in the hobby and how easy the gear is to set up. The HEQ5 is significantly lighter than the 6. If you'll be setting up frequently rather than permanent pier/observatory weight/size is a significant issue. The telescope you use the most and get best value from is the one you can be bothered setting up!
If you know you'll be in the hobby a long time then more expensive quality purchases generally pay off over time (versus constant upgrades). Unfortunately time travel is necessary to clarify that last bit
