My guess is....
Platinum => breakfast and lounge access
Gold => 4pm checkout (a la SPG).
Gold won't be able to give you too much if there's 3 solid tiers above it.
If this turns out to be true then Hilton will be a much better deal for leisure travellers and people who aren't heavy stayers. 20/40 stays/nights gives you guaranteed breakfast and a good shot at lounge access (outside the US) whereas the same in the speculative MR/SPG program might only get you late checkout. And HHonors now has elite night rollover while the combined program won't.