Just a slight correction in that you get the hotel's Standard Corp rate when you use the Plat override. And at FS hotels, that can be significantly below their "rack" rate.

To me, at least in NA, a Westin just seems like a FS Marriott without a lounge. And while Sheratons do have lounges, they don't seem to be quite a nice as the Marriott ones. In my NA stays, I don't think I have ever been upgraded to a suite room at a Sheraton or a Westin (as a Plat now for about 2 years). I will admit my Westin stays have been fewer than my Sheratons though. I have been upgraded to some nice larger "corner" rooms, but that is the same type of upgrade I get quite often at Marriotts.

My thoughts have always been to look at where you will be staying (both for your normal building up points travels and for your vacation travels.) Then decide which chain has the hotels there that you prefer. Also, I definitely think having points in multiple programs is a good idea. One chain may not always have hotels where you want to go, but having multiple options to choose from can help make sure you have an option.