For what you were discussing up top, there is no difference as to what would be sent over the wire.  You may have a slight hit using the (string, object) overload due to the conversion required, but the time you're waiting on the wire would dwarf the extra delay.

So in this case, I'd just pick whichever one would give you the most readable code.  Besides, if you're adding enough parameters that you're going to noticably slow down the client app, there are problems far beyond the .NET Framework with your design.  [grin]