Let me see if I can help explain a bit why the OP and I might consider other brands.
1) Europe. Just take a few minutes to compare SPG's European choices with Marriott's. Specifically Italy and Greece, but you can also look at every European country. Just look at Venice; the single Courtyard isn't even in Venice proper. And how about Florence Italy, one of our favorite cities?
2) Higher end hotels. Until Marriott improved their ties with Ritz and added Edition and the Autograph collection, I felt that this was a weak area with Marriott compared to SPG.
3) Treatment of elites. High end SPG hotels are light years better than Marriott as far as how they treat top tier members. The funny thing is that I found it a lot easier to reach top tier status with SPG than Marriott.
The really nice thing about LT Plat is that I will be able to have status even if I only travel a few days a year.
I was a SPG Platinum prior to becoming a Marriott Plat, and now PP. Marriott is a better choice for lower end hotels, but they fall short on FS hotels. I can tell you that going to a Westin, W, St Regis, or the Luxury Collection as a Plat, I was treated better and saw a MUCH higher percentage of suite upgrades than I have at Ritzs, JWs, Marriotts, and Rens.
In spite of having ~1/4 the nights in SPG hotels, I know that I've gotten more suite upgrades than I have during my time at Marriott.
I'm now at the point where I prefer higher end hotels. I'm pretty much done with FI, CY, TPS, RI, SHS, 4Points, Element, Aloft, etc. Even Sheratons are dicey in my book.
If Marriott's got a good hotel in the city where I'm going, I'll probably go there. But I'm now at the point where I'll take a SPG hotel if it's better than what Marriott offers.