Okay, you made me curious, so I did some more research :-)
I think chances created and assists can be summed up best with the xA stat (expected assists). This means you get credit for creating an opportunity for someone else to score, even if they miss the chance, and you get more credit for some assists than others (for example, a 5 yard sideways pass 30 yards out that someone else slams into the top corner isn't really the same as a perfect through ball that creates an open goal tap in). Over the last three seasons, Sterling has one xA every 400 minutes, whereas Rashford has one xA every 600 minutes. Last season Sterling was one xA every 300 minutes and Rashford was one xA every 400 minutes.
xG (expected goals) is a reasonable measure of misses, since it gives the difference between how many goals an 'average' player would score from every shooting position, and how many a particular player scored - so if you miss more chances than average, you score less than expected and your goals tally is less than your xG. Over the last three seasons, Sterling is 3 goals below his xG, meaning an 'average' striker would have scored 3 more from exactly the same shooting positions (so he's scored 91% of the goals he 'should' have). Rashford is 2 and a bit goals above his xG (he's score 15% more goals than he 'should' have). If you just look at last season, Sterling was a little better, but still only scored 96% of the goals he should have, whereas Rashford again exceeded the average, scoring 9% more goals than he should have.
The last stat you asked about was key plays. Sterling has had a higher number of key plays per 90 minutes in each of the last three years (1.91 to 1.10 last season, 1.64 to 1.07 in 16-17 and 1.62 to 0.62 in 15-16).
So whether you consider just last season or the last three, Sterling is better in the combination of chances created/assists and also makes more key passes, but Rashford is the better finisher (or you can draw your own conclusions based on any number of mitigating factors either way).
(all data came from understat.com)