It is a very personal choice.
Some Fellows who did the Fellowship during their leave said they valued being able to have non-baby related conversations during their leave, and that the peer group supported their return to work.
Plus they were able to use the learning from the Fellowship to shape their work pattern from the start upon their return, which can be easier than changing your arrangements 2 years after you returned from leave. But, you may also want to focus on just your baby during your parental leave, and that is ok too.