Unless I am missing something, PFA types can't do IFR and therefore there is no point in buying a Mode S transponder until they become mandatory; the current date is 2008 but could change.
What I would do if I had a plane of any sort is to buy a used Mode C - there will be a fair few of them about after 5/2005. That way I can be seen on TCAS and radar, which has to be a big plus for both myself and everybody else.
The avionics dealers currently installing Mode S units are stocking up the used Mode C ones, hoping to flog them to dealers in the USA.