They both are similar, but Martinelli is younger than Trossard, that's might be the reason because Martinelli could be more better.
It's already hard for 30 years old striker to play in Premier League, if he care with his career, I guess he will move to Serie A.
Trossard isn't for long term prospect, imagine if Arsenal successfully bring Gyokeres and Eze, it makes no room for Trossard.
You make a point about age being a factor but I still maintain that in terms of performance, Trossard is much better than Martinelli. The Brazilian has pace but often runs out of ideas in front of goal, unlike Trossard.Now that the procurement of Eze is still in the wind, it would be best to retain the Belgian because it would be unwise to throw away the dirty water you have just because you feel there would be rain.