For me, it would be Baines.
We have Oviedo already in the squad, and that means we can immidiately cover Baines' position with 'another' attacking Left Back. Whether he proves to offer what Baines does remains to be seen, but the early signs are there that he is an exciting player with speed.
We can then use the money to bolster our midfield.
Baines is 28, where as Fellaini is 25.
The only issue I have is that Baines is English. He is our dead ball specialist too, but there are others also, such as Jelavic and Gibson, not to mention Hitz.
If we lose Fellaini, we also lose an icon.
He can put us on the world wide map in the same way that Beckham succeeded, and the financial opportunities are huge, especially if Belgium do well. Without him, we would have a big hole in the team, that I feel is harder to fill.