As you have guessed there are some previous threads that discuss this. However as long as the thread is open I will put in my two cents as I am in the positon of being SPG Plat and getting comped to Hyatt diamond late last year.
In brief:
SPG pluses:
More properties available in more areas.
Suite upgrade guaranteed based upon availability (suites are much harder to come by at Hyatt).
Guaranteed (well sort of) 4pm check out.
Easier to earn points outside of program
Ability to take points for the amenity at most properties.
For the most part room rates are cheaper.
No blackout policy (as long as there is inventory in the "regular" class of room).
A lot more redemption bargains out there.
In general much more comfortable beds. I am not sure why but most Hyatt properties seem to have an uncomfortably hard bed (the Maui Resort is a notable exception)
Hyatt pluses:
More devotion to customer service than SPG (don't get me wrong, for the most part SPG does a great job, but Hyatt is amazing).
Ability to get stay/night/promo credit as well as elite priviledges for award or non-Hyatt channels (including priceline) as long as some incidental is charged (e.g. a phone call).
Free breakfast if no club room.
Amazing promotions - e.g. Faster Free Nights.
Does not seem to engage in targeted promotions being denied to non targeted elites.
I plan to requalify in both programs, but I have to say that I favor Hyatt a bit. If everything remained the same and Hyatt had a credit card to earn points with, I would consider letting the SPG Plat lapse.
One thing that really conviced me was the week I spent on Maui in October, I spent time at the Hyatt (Faster free nights) and then some time at the Westin and Sheraton. The differences were amazing on many levels.
At one point I even went back to the concierge at the Hyatt (after we checked out) as the concierge at the Sheraton seemed disinterested in helping me out with anything that didn't involve selling me a ticket to something (The request was for a used cardboard carton for shipping some things back to the mainland. Sheraton's response was "You might try calling operations."
. Hyatt's response was "Give me a couple of minutes to see if I can find something", followed by "Here you go, sorry for the wait. Will you need packing tape? Please feel free to bring the box back here and I can make sure it goes out with our UPS pickup." The Hyatt concierge earned both a tip and a bit of brand loyalty.
On the other hand if you do a lot of travelling outside of big cities, you may want to stick with SPG.
[This message has been edited by VPescado (edited Feb 24, 2004).]