I made the same decision about 12 months ago and went for PC.
Factors: better availability (ie. more) of their hotels, at a better price, for the destinations I was more likely to travel to.
Whilst it hasnt worked out 100% (I ended up at Grand Hyatt in NYC due to exceptional rate), I have been happy with the decision.
Note both are free to join, so I joined both, then went looking at accom options for my upcoming trips. As it turned out, PC-hotels had the best pricing/location/availability and hence easiest to stay with the one chain and earn points.
Their redemption, although I havent used yet, seems simpler too so that's a bonus, and as others have mentioned, they seem to have regular "points-breaks" (ie. discounted "award" rooms) at multiple locations which ehances our ability to use points. There also seems to be multiple "bonus" offers with PC so all in all, I'm quite happy with the decision to go with them.
YMMV.