I agree Sterling would be the safest option even with the bigger price tag. However, myself being a gambler, I would go for Rashford.
Reasons: I'm convinced we are not getting his full potential at utd and there is a lot more there than we are seeing. Which would, of course, narrow it down to the people looking after him. It would be realistic to say that Sterling has improved a lot under Pep's man to man management. Rashford had already shown what he was capable of when he burst onto the scene under LVG. IMO Jose with his onus on defensive qualities has stifled his development! Under Pep, for example I think we'd see another Rashford than we are seeing today.
Of course I don't have facts and it's only what I believe, and yes I'm taking the liberty to have a dig at our manager. That said we have seen traces of a very gifted footballer we have on our hands, which was there to be seen in his latest performance for his country. No problem with decision making there! And no I'm not building my judgement on one game!
So in answer to the OP, I'd go for the 20 yr old Rashford and turn him into the player he should be!
If I was Sarri or Poch that is!