Marcus is a hybrid. He’s got great pace, is a good finisher when on his game. A bit too slight to be a single striker. Skilled, but not skilled enough in tight quarters or to take on multiple defenders like Sterling or Sancho.
Tammy is a classic lone striker who has all the tools to hold the ball up, is good in the air and is a good finisher/poacher. Basically a Didier Drogba clone. Doesn’t have as much pace as Marcus and isn’t as skilled with the ball at his feet.
There is a chance, a very small one, but still a chance, that Rashford develops to have a Thierry Henry type of career, starting out as a winger, then becomes a goal scoring forward. He will get stronger, and with better players around him, his goal scoring will improve, so it is possible.
Between the two of them, Rashford has a higher ceiling, but is less likely to hit that ceiling.