Martial > Griezmann
1. Griezmann will likely cost us at least 250-300 million over 5 years (100m transfer + >30m gross wage now that Atletico offered more). There were also rumors that we would pay Atletico 120m instead of the 100m buyout to avoid hurting our relationship with them which would increase the cost even further. Martial is appearently only on 65k per week at United, so would probably only cost about 100-110 million over 5 years (60-70m transfer + 5-8m wages)
2. It's impossible for Messi, Griezmann, and Dembele to all play together in their best positions. Dembele or Griezmann would have to play on the left, where they are not nearly as effective.
3. Martial is younger with tons of potential and would not be a guaranteed starter every match, which would make it easier for Valverde to rotate between him, Suarez, and Dembele.
4: Most importantly, it would be amazing sticking it to Mouyes if another one of his talented castoffs went on to become a world class player for us.