I've typically held Gold/Plat with all of Marriott, SPG, and HHonors.

Starwood does, I believe, now offer a challenge. One advantage for it is that it's well-suited for your one-night stays. (See their board for more details - I think it's a sticky.)

My HHonors upgrade rate was a little better than Marriott but nowhere near Starwood, where I'm in a true suite more often than not. For your priorities, including strong properties internationally, I think SPG is a very good choice.

Only place HH beats it (IMHO) is that they seem to prioritize lounges in general higher across their main brand. Only Sheraton (of Starwood's primary brands) is highly likely to have a lounge, and frankly some of those lounges are poor - a cut below the typical MR or HH lounge. Honestly, I've grown to like my regular Le Meridien: they have no lounge and give Plats a full buffet breakfast.

It sounds like hitting 21 to 25 stays per year (even if they're all one-nighters) shouldn't be too hard for you, meaning you can easily requal Plat even if you continue to enjoy your Marriott LT Plat a large percentage of the time.

So I'd probably lean SPG but wouldn't think of HH as an awful choice either. I like having access to all three at at least a mid-tier level. With your travels, you could probably even do 21-25 SPG stays, use the HH Amex as your primary spend card, and wind up with THREE top-tiers...but that might be overkill.

And I agree with Sharon, don't stretch yourself so thin chasing statuses that you forget you still have a nice Marriott lifetime Plat to enjoy.