Finishing-wise, Greenwood is better than Rashford but that's not really the bar to compare to.. Greenwood is clinical compared to the vast majority of forwards.
I think Greenwood is a better finisher than Martial, whose fans here call him ice cold and clinical.
However it's still not clear that Greenwood is a better player. We've had entire seasons where we counted on Rashford, and none where we've counted on Greenwood. He's been able to play selectively and make an impact. While Rashford has more suspect decision making than Greenwood, Mason more often just disappears from games when not making an impact.
We'll see soon enough, but they're young players and have plenty of time ahead of time. It's too early to call who is better, but it's easy to see who has done more (Rashford).