Well Mark, as I said in a message below, I managed to get a Leopard out to Newtownards, this was 183 (VOI 183) on service 5 from Belfast - Newtownards. This particular service left the Laganside bus centre at 14.15 and I think this service is operated by a Leopard most days. If you travel on this service, it is unlikely that you will get a Leopard on your return trip. You might get one of the new Scania Solars. In a way that makes this service very interesting as it is operated by buses at the different age ranges.
As for Coleraine, lots of Leopards and a couple of Bristols. I take it you will be staying in Belfast, so if you wanted to go to Coleraine you would need to get Goldliner express service 218 to Portrush. As Kevin said the biggest fleet of Leopards is found in Coleraine. Alot of Leopards have been transferred here from other depots during the last couple of years, eg 174 & 281 from Larne, 116, 240 & 333 from Laganside and 232 , 268 and 271 from Omagh.
You best bet to get a CL Leopard is if you travel at peak times of the day, but most school buses in CL are very busy and it is often difficult to get seats.