Everything in C# language is an Object, there are no primitive types in the classic sense.
That means string is not a truly primitive types, it is an object instance of a Class.
System.String and string are compiled to System.String in Intermediate Language (IL), so there is no difference in the performance, and it's purely a personal choice.
Also;
string is an alias for System.String. They compile to the same code, so at execution time there is no difference . This is just one of the aliases in C#. The complete list of alias is:
object: System.Object
string: System.String
bool: System.Boolean
byte: System.Byte
sbyte: System.SByte
short: System.Int16
ushort: System.UInt16
int: System.Int32
uint: System.UInt32
long: System.Int64
ulong: System.UInt64
float: System.Single
double: System.Double
decimal: System.Decimal
char: System.Char