I have been the sound editor on an animated series about fairies as well as a series involving giant robots so I have a history with both these options. The giant robots are much more engrossing and involve lots of elements while the fairies are all about soft sounds so you don't use as many layers but the ones you do use have to be bang on perfect, because there is no where to hide a sound that is not quite right. So They are both difficult and engrossing in their own ways. Over the long haul the fairies started driving me crazy though. Due to their small stature the sounds end up mostly residing in the higher end of the sound spectrum and fairy voices are annoying as hell (at least on this series). The robots become very fatiguing on the ears after weeks of the metal clangs and hits, so I guess they both have their downsides as well.
One interesting side observation. With the fairies each fairy had to have its own full library of sounds. Each fairy had a completely different sound design for their wings flapping, no sound was used for multiple characters. While the robots had many signature sound elements per character, but there were also many sounds that were involved in almost all the robots, ie metal rings, crushes, and hits. Now this has as much to do with the character design on these two shows but I think illustrates that the fairies were all about the details and the robots were all about conveying their hugeness with big loud sounds.
I played with robot toys growing up but never had any fairies kicking around, so in the end I guess I would lean towards the robots because they are more towards my natural interests.