I stay at both Marriott (PP and LT P) and SPG (P and LT G) extensively. I would put the differences in a few categories based on my experience which are important to me:

Upgrades - I tend to get much better upgrades with SPG. I have gotten some fabulous suites through SPG (many before Suite Night Awards were introduced but that is a whole different topic...). I very rarely seem to get full Suite upgrades with Marriott with the exception of travel in Asia and Europe.

Points - Even though Marriott has increased the redemption value of many properties, I still find it much easier to earn points with them. I earn 20-points for every dollar spend (10 base + 5 for Platinum + 5 for MR Credit Card). Even though categories are much higher then SPG, I still seem to earn and redeem a lot more in MR than SPG. I am not impressed by either Marriott's Unexpected Bonus or SPG Double Points promotions this summer.

High-End Properties - SPG wins hands down. I get very little special treatment at Ritz-Carlton (limited upgrades, no club access, no points earning other than base rate). SPG offers upgrades at both ST. Regis and Luxury Collection (I just got a fantastic Deluxe Suite at the St. Regis Punta Mita) and full points earning on every dollar spent.

Geographic Coverage - Marriott has hotels in almost every city and country I need to travel. There are a number of US cities and countries where SPG has no presence.

Guaranteed Room Type - Marriott hotels guarantee the bed type which is important to me. Only once in over 1500 nights of stays with Marriott, have I not gotten the bed type I reserved and I got $100 cash for my trouble. I will sometimes get Double Beds with SPG even though I reserved a King Bed.

Based on the pros and cons, I will continue to stay at both. I still need one more year of SPG Platinum status to get LT Platinum.