Hi I played until lvl 60+,
the goldball is very strong as is the green ball. You kind of need many green balls to even get the chance to buy more gold balls later on as the max amount of gold is 150 and each goldball will cost 130 so the next round your gold balls will be very weak.
Ultimately the green ball is the strongest as the Max HP keeps rising.
Both balls are just OP to be honest but I love the concept of the game anyway. Getting more damage from rising my HP and strategizing over when to risk to replace another ball was very fun.