Personally, I think the smartest way to pick your program is to look at the course offerings and make a decision based on what you'd be most interested in taking in your upper years.
If you're more interested in one programs' courses, maybe you should do a single honours in that program. If you're kinda half and half, do the combined.
Consider doing a minor in one subject if you're not interested in the requireds for an honours or combined honours program. I know poli sci has some requireds that are kind of a drag if you're not interested in that field of poli sci (ex: political theory, canadian politics)
All things to consider
Here are the course offerings:
Sociology
http://registrar.mcmaster.ca /CALEN...nt/pg1845.html
Political Science
http://registrar.mcmaster.ca /CALEN...nt/pg1835.html