I have no idea with websites.
In my opinion, main differences are;
Elite Qualification:
SPG: Based on STAYS/NITES
PC: Based on ALL ACCUMULATED POINTS/NITES
Qualifying Rates:
SPG: Nearly every rates that paid at the hotel
PC: Unclear. Most of web booking rates are okay
Top-Tier Benefits (published status only):
SPG (Platinum): 500 or 250 points bonus every stay / Always access to Club Lounge at Sheraton/ Westin
PC (Platinum): 50% bonus point / NO guaranteed lounge access in T&C
SPG is okay, but PC seems to be suited with my travel way.