To answer the question - I think it was probably about 50/50 until a couple of seasons ago.
The number of fans who have followed the team from Shawfield/Firhill days is definitely dropping quickly these days though.
The main reasons IMO are :-
1) Fans getting older. Less enthusiastic. Can't be bothered travelling any longer. Kicking the bucket (Unfortunately. I know a few who are no longer with us).
2) Too much attachment to one half or other of the Old Firm over the last few years. The last 4 managers have all played for either Rangers or Celtic, as have several coaching staff. That doesn't sit too well with a lot of supporters who have no time whatsoever for the Old Firm.
3) Poor home form, especially after 3 or 4 years where it was excellent.
4) No longer the feeling at the club of being "one big family". Again, my opinion, but I think this started when Graham Roberts took over, and he tried to make the setup more "professional" by clamping down on supporters having access to certain areas of the stadium. Ok, you can argue it wasn't right - but it was what made us different to other clubs.
5) The Board have alienated themselves from the "ordinary" fans by making unpopular decisions without giving a reasonable explanation for their actions e.g. Graham Roberts sacking (case lost at tribunal). (Personally, I think the Board have done a good job (considering our small support, and financial problems) to keep us in the First Division - but I know many others don't agree with me).
The number of Cumbernauld fans has probably remained fairly static in the last few years (when we need it to be on the increase).
Natural progression at any other club is for new young supporters to start supporting the club. Unfortunately this doesn't seem to be happening with Clyde as much as with most other teams.
Currently I'd guess 40% Rutherglen/ 60% Cumbernauld - but only because the old fan base is dropping away rather than the new one increasing.