If I had to choose between the two Id choose Greenwood by a fair margin.
Both are great talents, Martinelli is faster and has supperior dribbling and has performed way better than anyone could had expected. the jump from Brazilian 4th division directly to the premier league is huge and he has surprised a lot of us. I think he can be a really good player.
Now after saying that I think Greenwood can be a generational player, he is a stiker and has given solid performances as a right winger. He has the calmness and is overachieving his xG times 2 that just speaks at how composed he is about finishing thats a really special quality. Even if he didnt had all the other attributes the finishing alone could make him a really valuable member for any top side.
At the moment a clinical strikers are more valuable than a tricky wingers, almost all the top ones are aging Lewandowski, Aguero, Suarez, Vardy. While there are lots and lots of good wingers.