Set up and sailed similarly, the j24 is a bit faster. As the breeze comes on they are much faster.
The J24 is generally better build quality.
There is better tuning info available for the J24, to help you get the best out of the boat.
The S525 is probably considered better looking by most people.
Interior layouts are very similar.
You ask the merits of both. Funny you used that word, since the Merit 25 is better than both, if you can find one.