I only played with the joola practice p 40+ for a few hits and can't really say anything but our club uses the Butterfly Easy Ball mainly for robot or if no 3 Star Bally are around. (Mainly cause someone forgot the key to the locker).
The balls are pretty round. No 'eggs' in a box of 120, but lookin' quite almost hollow. But they play better than they actually look, but of course they aren't three stars but totally okay for the price. That box of 120 cost less than € 50,-
But on the other hand: when we are doing drills or playing game drills we mostly use our three star balls. Cause we like to follow the unwritten rule:
Practice how you play!