There is no difference.
DefaultRequestHeaders.Accept is a collection of string type, where you can add your header to accept using the new instance of MediaTypeWithQualityHeaderValue.
client.DefaultRequestHeaders is a dictionary that accepts key for and value for the request header and matches the results according to them.
DefaultRequestHeaders
has overloads.
The only thing that differs between them, is the fact that DefaultRequestHeaders.Accept will require you to initialize a new instance of MediaTypeWithQualityHeaderValue class, resulting in another reference type in the heap, while client.DefaultRequestHeaders will add the data to the dictionary, removing the cost of resources and the need to initialize a new instance.
It is really up to the user as to how and what to use.