I have no idea with websites.
In my opinion, main differences are;
SPG: Based on STAYS/NITES
PC: Based on ALL ACCUMULATED POINTS/NITES
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.
PC allows you to have less expensive stays - most SPG properties are quite pricey. Which is okay, if it is paid for, but not on your own dime.
There are some special deals which make them less expensive but in general, they are pricey. HI / HC / CP are all the same, while the SPG properties seem to be nicer. Well, I think they should be for the price difference.
SPG does cheat on the way they present the reward stays - yes, you need only a fraction of the points needed with the stay, but you also only earn a fraction of the points ...
spg itself has an overview on its page for some parts of the comparison.
For myself the reasons to go with PC are the following:
I am biased towards liking the IC. *g*
The have a good coverage in Germany/Europe, instead of just America. I put in some of my likely stays and found some interesting stats for my own plans.
Prices are from higher to the lower end. I prefer clean over pricey but fancy while on my own dime and spend this money elsewhere, for example the IC. Most SPG properties - while probably nice - do cost more. Period. If you need a bed for a night and not a luxus stay, then you will have trouble finding a lot of decent priced SPG.
As I said, sometimes there are goodies, so it is not bed to consider them. When you do reach Platinum, it is probably nicer to stay at SPG and they are more consistent throughout some places especially the US.
SPG Plat is certainly a better Elite level with more consistent and meaningful perks than Plat or Ambassador. Point earning at Plat is pretty easy with the 50% bonus and 500 amenity points.
But as both posters above comment, which works for you is driven by how much you spend and where. If you are a mid tier in spend and primary in Europe then PC makes much more sense. If spending more in Asia and the US, then Starwood offers better value. Certainly PC has better, more mid tier (i.e HI) style properties.
Also, are you interested in awards or elite benefits? That can make a difference.
Award availability in Starwood is much, much better. I have never had any problems redeeming with SPG whereas the desirable properties in PC are tough.
Programs: BA G, TK *G, UA *G, SPG Plt, PC Plt, Marriott Gld, Hilton Gld, etc
note that top PC property (IC) costs 30,000 points (always) which will be yours for $2,000 spent (without promotions, including plat bonus). top SPG properties will be yours for 20-25k points which will be yours for $6667-$8333 (without promotions, including plat bonus).
another consideration may be that number of properties is ca 800 for SPG and ca 3500 for ICH.
Lastly SPG properties overseas (in Europe for example) tend to be represented by higher cost properties (hard to find anything below category 3).