Soap Response performance

I have a soap webservice extended from  %SOAP.WebService class

Lets say i have web method and have return type of following classes (either one of them)

Class ReturnTypeA

{

  Property SelectedDoses As %XML.DataSet;

}

Class ReturnTypeB

{

  Property SelectedDoses As list Of WebServices.DataTypes.Dose;

}

Which return type class has better performance while in terms of xmL serialization ? A or B

my client application is in C#