Cost would be the same and coverage mileage and time frame would be the same if purchased now or before 36,000 miles or 3 years from date it drove off the lot. The paperwork shows what is covered and that can be obtained from the source. Platinum is the top level and has no deductible.
Thing is, these deals can dry up at a moments notice. My first source I had about 5 years ago DOUBLED the price over night, so I had to find a new source.
Sounds like you don't put too many miles on vehicles so I would go with longer term vs more miles.