Psychologically speaking losing Baines, especially to UTD is a bigger blow than Fellaini.
He's seen as one of us, and losing him to UTD after Rooney and our manager, only serves to further emphasize our 'also ran' status. The idea that we will ever compete at the top again is already tenuous enough.
Selling 'foreign players' is kinda expected at some stage.
Financially speaking, selling Baines for circa 17-20m is Better business because of his age and the fact that he's a full-back. You could keep Fellaini for another year and sell him for the same price.
For Martinez, it's a balancing act, he'll want to hold the squad together so that it doesn't send out the message that the wheels are coming off but he'll also want to bring in funds so that he can build-on and establish his own style.
If he sells either, or both, he'll need to make a statement by bringing in a big name.
Hold onto your hats...