I agree that Martial would be the more obvious player to sell over Rashford but why would we need to sell either of them? We've finally got some depth in the frontline so I don't know why we'd look to tamper with that, unless we were looking to replace Martial, which wouldn't be a given.
I disagree that Rashford's outperformed Martial over the period before that, their stats for that period are again very similar, and they were both in and out of the team a lot under Mourinho so struggled for consistency. Rashford also was first choice backup for the CF position under Mourinho, and an out and out starter in that position under Ole in his first 6 months, so I don't think the excuse can be used that Martial plays upfront so you expect him to have better stats. It's all subjective but it wouldn't be unreasonable to suggest that they were at a similar level until this season, despite the recent narrative on here.