There are 2 different levels, not including Platinum Elite Lifetime.
Platinum is for 75 nights or more. Platinum Premier is a reward of a certain top percentage of group, but generally would focus on people staying probably more than 100 nights. The criteria for that level are pretty much undisclosed for a certainty as to say who will be chosen for that level. Can't say that it counts for anything more than a special gift at the time that they are chosen, around March of each year. They are considered a bit more for special consideration of upgrades, too, it seems. No real certainty other than that there are 2 platinum levels.
As a Platinum Premier, when I log onto Marriott member site, I can see the PP tier level mentioned on several web pages. A colleague who is Platinum not-Premier does not see them displayed. We had our screens open side-by-side and Marriott is providing Member-level specific content even on the website if you are signed-in
I did overhear a Manager at a Property state that they were overbooked. There was a Platinum Elite member at the desk trying to get a room. She was not able to bump anyone for this member. She did however tell her desk manager ( after the member left ) any time a Platinum Premier member showed up that they would always get a room. So far that is the only true difference I have heard.