in my opinion, the only reason for getting synthetic over leather would be cost, so if they cost the same, i'd go with the leather.
a lot of it depends though on personal preference. if you're buying the shoe for a kid whose feet are still growing, you won't see the 'mold to your foot' benefit of leather because he'll be in new cleats next season anyway.
the only real benefit to synthetic is that they're usually water-resistant and might hold up better in soggy conditions. and because they can get wet, they're much easier to clean too.
the advantages of leather are the gradual molding for better fit and feel and they're more durable over the long run.