Ocellaris and percula are reputedly two of the more peaceful species of clowns. I don't know that one is easier to keep than the other, just look for tank-raised specimens if you can find them. I have an ORA misbar percula in a 20 gallon that my husband and I got because we like his messed-up stripes.

He's been really mild mannered in our tank. While working at a
LFS, I have observed a TON of ocellaris clowns and they seem very similar to the perculas in temperament. I believe the ocellaris stay a tad smaller than perculas. Here's our misbar perc if you've never seen one:
Meet Wiggles :)