I thought Ekholm had a little bit of an off year last season, but not because he had slowed or due to any physical deterioration, just... he wasn't quite "on" his game especially in the first half. But he still seems to have all his tools intact and I expect a return to top form for him in the coming season.
Plus it will help him that his likely D partners, whether Fabbro or Carrier, are becoming more experienced and that should ease a bit of the burden he has carried since Subban fell apart. They were a tremendous pair for a couple years, but it feels like Ekholm has been leaned on to really carry the 2nd pair ever since.
I haven't watched McDonagh closely enough to be sure, so we'll see, but I would expect that Ekholm will continue to be a notch above him. He has mobility and offense that McDonagh probably can't match, and they should be equally solid defensively.
So: Ekholm.