Any large hotel chain is going to vary from property to property.

In our experience, as Platinums with Marriott, we get upgraded at least to the concierge floor 99% of the time. Suite upgrades are less frequent than they used to be, but they do happen. A good friend, who is mid-tier (Gold), is often at the same conventions we are. I'd guess his concierge upgrade success is 50% - but he always is given lounge access without any trouble.

We used to keep Hyatt Platinum (=Marriott Gold) as well as our Marriott status. I can think of exactly 4 times in 6 years that we were upgraded to the Regency Club. NOT exactly a great track record. Hilton, where we had no status for years because they seemed unable to credit most of my stays, upgraded us as plain Blue card members more often than that. But we finally just got totally disgusted with the Hyatt attitude, which is basically "Oh aren't you lucky to be staying in our wonderful hotel, you peasant." Or so it seemed to us.

Hyatt tries to pretend that it's more upscale than Marriott, and indeed there are a lot of cookie-cutter 3star Marriotts out there, many fewer such Hyatts. But we're not talking Motel 6 here. And the high end Marriott hotels are certainly as nice as similar Hyatts (I don't want to get into too much detail, comparing Park Hyatts, Grand Hyatts, JW Marriotts, Renaissance, Ritz-Carlton and so on). And frankly, especially when I'm travelling on business, give me a humbler hotel with great staff over a fancier building with attitudinal staff, thank you.

You need to look at where you're going to be travelling. As silverpie points out, there are far fewer Hyatts than Marriotts or Hiltons.

btw, we are now very happy with Hilton (the customer service is 1000% improved) as our "other" chain. It's pretty easy to get status with Hilton, too, although upgrading and lounge access seem to be a little spottier there than at Marriott. But we're just mid-tier with Hilton, although it's possible we'll make Diamond this year.

The thing about Marriott, as you can check for yourself if you do a Search on something like "comp status", is that generally you have to earn your status by staying the nights. This is especially true of Platinum - but it's why that status is still pretty valuable.