I wouldn't turn down a contract offer if i was wan bisakka. Hanging around for a year, running down your contract, trying to avoid injuries isn't a great platform to sell yourself on. Look at Lingard. Look at De Gea. Better players than him have struggled and retired pretty early. Maguire is a better shout but he's had a few injuries this season, its gone under the radar a bit because others had more and longer lasting injuries. I'd worry more about where they're going than how much of their wage they'll keep in both cases.If I was him I’d stay ant United for another year and then move on a free.
Same advice to Maguire, he has 1+1 year remaining. United have put themselves in a bad place after years of bad dealing, selling them or negotiating new contracts, they are going to get exploited either way.