I travel almost every week and have for the past 15 years. Over the years I've had Platinum status with Marriott for many many years (still have over 500,000 points yet to use), Diamond with Hilton (on an off for a few years) and Platinum with SPG.
My ranking of the high-end programs are:
SPG, Marriott, Hyatt (just not enough of them around) and then Hilton
IMHO, SPG points wins hands down. Really, having used all 4 of those programs to the fullest, SPG really is that much better.
-A few examples, if you want to use points with SPG, no blackout days and it has been that way for a while. Marriott only recently went to that option.
-SPG has a fantastic option called points/dollars. Instead of using say 10K points for a Category 4 hotel (ie, high-end Sheraton or most Westin Properties), you can use 4000 Points and $60 per night...This is a great way to extend use of points and get a huge bang for your buck.
-One important thing for us business travelers...upgrades. Marriott's idea of an upgrade is typically an upgrade to club floor; on occasion they will provide an upgrade to a better room (ie suite). SPG on the other hand (for Platinum members), will almost always upgrade you to a suite. Many of us use our points on family travel so this perk alone is a HUGE benefit. It is so nice to have large accomdations/better views (ie oceanfront suites) when traveling with family. Over the years of being Marriott Platinum would have to call before hand and beg for a decent upgrade...with SPG they do it automatically. Case in point...just this past weekend we stayed at the Swan (Westin Property) and used free weekend vouchers earned from their Summer Promotion. I checked in periodically over the past couple of weeks at SPG.com and kept seeing my room change..it went from a standard room, to deluxe room w/balcony, to deluxe room w/balcony and theme park view, to a deluxe alcove w/ resort/park view to my actually checking in and getting deluxe alcove w/balcony and a park view...essentially received 4 levels of upgrades...I can tell you now that ain't happening with Marriott (for full disclosure on special trips where we were staying in Marriotts, ie, birthdays, anniversay, if you call the hotel your staying at and speak to a manager and ask nicely, they will almost universally go out of their way to give you a true upgraded room.)
-One last thing you only need 50 nights to achieve Platinum status with SPG, Marriott is 75, and Hilton for Diamond is 60 (I do like the way Hilton does the timeframe though on a rolling basis as opposed to all others on a straight calendar year perspective).
-Beds, SPG has the Sweet Sleeper Bed...the other's don't...reason enough right there
Just my thoughts...