I have an optimistic bet on Ajax, halftime/fulltime Ajax Amsterdam/Ajax Amsterdam at 2.43! Ajax is like a machine, they are winning from week to week, 6 wins in CL, and I don't think they will stop here!
I also feel they are strong favorites tonight, they have scored 70 goals in Eredivisie and conceded only 5 times in 20+ games.
However - when they did concede, they usually lost the game. They have +65 goal difference in 23 games but have lost 3 and drawn 2 of them - meaning that they don't know how to turn things around when it doesn't go their way. Benfica is not the team it was but still finished 2nd in the group with PSG and Barcelona, meaning that they won't be pushed over easily. Still, I feel your Ajax bet has decent value.
I know it's an optimistic bet, Benfica is not a team we should underestimate, and they are playing at home! I just think that Ajax will continue with their aggressive style from the first minutes, and if they score some goal in the early game they will lead the game till the end and they will win! But as you say, if things don't go that way, if Benfica surprise, which may happen with some counter-attack, Ajax will not be able to turn the game in their favor!
Well, in my opinion, this will be a more interesting game than Atletico vs United! But we will see what will happen! Good luck with your bets!