As far as different treatment between Gold/Plat, I really didn't notice much difference for stays in NA. If you have a lot of non-NA travel planned, it is possible that you might get some better upgrades as a Plat. But other than that, I never noticed a strong difference in treatment.

As for general benefits of Plat over Gold, as you mention you will get an arrival amenity and more points. Also, Plat gets a little better elite benefit Guarantee and the 48 hr guaranteed room availability.

The amenity is usually some points (between 200-500pts depending on the hotel brand), or some snack items. At non-FS hotels it might be a bottle of water and some cookies or possibly a choice of one or two items from the hotel's little grocery area. At FS hotels, you might get options like peanuts/beer, ice cream/milk or soda, half a bottle of wine/fruit or something along those lines. Unless I'm with the kids and they want the ice cream or sodas, I usually just go for the points.