Both are great players! The price payed for Mbappe seems a bit overrated for a player with 18 years old, about Martial he's getting better match after match it could be happening because of Man Utd have more players speaking french than the last season. Another point that we have to consider here is the fact that the Premier League is one if not the most competitive Leagues in Europe, we actually can't say that Mbappe is a super star, he's young and need to prove his talent in other country for beeing a star in my opinion.
I would like to ask everyone what do you think about the portuguese Gelson Martins from Sporting Clube de Portugal, he got some rare skills sand he's fast as a flash you could take a look over there :