The position on the arc indicates how much more you can power up the Pokemon at your current level. Notice how there's more space in the arc of the top Goldeen, yet its combat power (CP) and HP are still higher than the Goldeen below it? That means it has better stats, and will have a higher potential maximum CP (ie. when you've powered the Pokemon up until the arc can't be filled anymore). So the top Goldeen is the better choice.
Verifying this with the IV (stat) calculator:
https://pokeassistant.com/main/ivcalculator
The top Goldeen has an IV ranging from 55.6% perfect to 84.4% perfect. The bottom Goldeen has an IV ranging from 28.9% perfect to 44.4% perfect.