I won't vote because I feel both are equally important. Look at Liverpool. Allison, van Dijk and Fabinho were the three signings that took them to the next level.
Now we're not gonna replace De Gea this window, and as patchy as De Gea has been, I wouldn't put him anywhere near Karius level. But I do feel we need signings of the calibre of van Dijk and Fabinho to win the league.
If I absolutely was forced to choose, I would choose a defensive midfielder. I have more faith in Maguire, Bailly and Lindelof providing an adequate defensive 'trio' for the season, with Tuanzebe as backup, as compared to just Matic as our sole CDM. He can't do it for an entire season, playing 2 games a week. The difference in the team when Matic plays, vs McTominay in that role is night and day. Not bashing Scott, but his strengths are more box to box, his aggression and athleticism is wasted in a holding role. Fred similarly isn't physical or tactically aware enough to play that position I think.
Sancho, and a CDM would be a good summer in my book. A CB would be fantastic too, but I don't see any CB that fits the bill for what we need right now. Maybe Koulibaly, but he doesn't fit the age profile Ole clearly targets, and I doubt we'd sign him and Sancho in the same window. Therefore, I would say we should get our no.1 targets this summer for RW and CDM, and maybe wait another year where the likes of Upamecano or someone might be available.
We should be targeting summer 2021 as the summer to get the final pieces in order for a real title push. Next season is too soon, and I'd rather wait and sign two top players this window, rather than Sancho and two 'lesser' players using the excuse that we need squad depth or numbers.