Easy answer for you: get both cards. Both pay for themselves and then some.

There are two kinds of hotel devaluation: structural program changes (semi-rare) and hotel category creep (annual).

Marriott's structure as a program has been pretty stable for a few years. The last structural "devaluation" (IMHO, about 7% for a Platinum member) was a few years ago when they removed the two-tier redemption. But in the "new" program, I've actually had better luck redeeming awards, so I won't complain about that 7% (The two-tier era was frustrating, because I'd frequently find a string of 7 nights broken up by one or two with no availability.)

The biggest risk to a structural devaluation with Marriott is if they try to do something creative when they ingest the Starwood members and properties into MR. That said, my bigger concern is for my 200,000+ Starpoints, which I personally enjoy using at about 3x the value per point as a Marriott point.

I'll worry about that in a year. Won't affect 2016.