It's totally up to you. The Celestine fairy has a chance to drop gold apples & nuggets as well as lumen essence, which I've used a lot of for this quest. So for me, it's an easy choice. I'm putting the mermaids on hold (except for collecting from the castle, etc)
The beauty of storyline is that it is always there. If you take a break to do a timed event, you can just pick up where you left off. And maybe you'll have a new addition that has helpful drops. Or even, gasp, another source for coins. Lol.
Yes, I admit it! I'm learning to appreciate the coin only droppers now that we are buying coral and anemones.
Good luck with whichever you decide!