I've been SPG Platinum for almost five years now. In that time I've noticed a marked deterioration in the quality of the experience. To some degree I think SPG Platinum has become a victim of its own success: too many qualifying members have led to a dilution.

I've had an Ambassador for the last couple of years but I don't think it's done much to alleviate the infrequency of upgrades, lack of availability or indifference of many hotels. SPG Platinum is so common it just doesn't really mean all that much nowadays. Within Southeast Asia I think Starwood is still unrivalled for its reach and quality of hotels. I'd rate North America eat the opposite end of the spectrum aside from a handful of (expensive) St Regis properties which are pretty good.

I switched some business across to Hyatt because of this. Hyatt promises less but I think they more frequently exceed my expectations, both in terms of service and also nice little unpublished perks here and there. In North America, Europe and East Asia, I think their hotels are of a higher standard than Starwood, but most of their Southeast Asian properties (with the exception of GH KL) are pretty old and dilapidated.

If I were in your shoes, I'd be making the decision about which program to run with based on the region you travel most to and also the region you aspire to spend more time in. Regardless of how good a loyalty program is, it can only do so much to alleviate a horrible hotel and by the same token, it won't do much to enhance what is already a great hotel that ticks all the boxes.

Or if you can, hedging your bets can always open up more options!