Web Api Restful service returning serialized string vs object

I have a question. I am creating a web service for the first time and I want to return a json. My question is, is there a difference if I serialize the List object in my code, using newtosoft, and return the string than returning the List object and letting the azure handle the serialization? I mean, do you see any real problem if I use the first method I described above? Is there a limitation in case the result contains a big number of entries in the List?

Thanks.