Ok, so the analytic geek in me came out. Attached is an excel spreadsheet that can be used as a general guide as to whether one should stay with the old Marriott card, or consider the new one. It is not an exact science, for sure. The new card seems to have significant benefits "if you DON'T NEED the extra nights earned for the $3K to continue qualifying for annual elite status". If I find out for certain that I can maintain platinum annually by only having 50+, which eventually gets me to 600 nights AND 10+ years (for LTP), then the new one might be good for me after all. But, if 70 nights (as the green column on the right in spreadsheet) means all I eventually get is LTG, then I will stick with the old. Still trying to decide.
The spreadsheet is color coded, and should be fairly easy to follow (hopefully).