Depends on the fee, but if it's Matic for £40m or Dier for £60m, I think United should go with the former. More experienced, proven winner, at his peak better than Dier is at the moment and at 28/29 he should still be there or there abouts, also he's worked with José before and I feel would fit the role and formation better.
I don't think either are good enough long-term, but at least Matic would only be a stop-gap. Dier would be a heavy investment and stuck on a 5 year £100k+ a week contract and probably end up needing to be replaced in 2-3 years anyway when United realise he'll never be world-class.
(But yeah, besides the point of the thread, United should have went for Fabinho earlier in the window when Monaco were more likely to sell, or even Krychowiak now).