Arsenal won all their group stage matches including big teams but it's always a different ball game at the knockout stage. I see Arsenal as the favorite to win the game. Football is full of surprises, anything can happen, we might see a different outcome entirely from our expectations. Only gamble with the amount of money that you can afford to lose.
Arsenal are undoubtedly favourites against Sporting. Arsenal have a better chance of winning. No one will argue with that. Arsenal are in good form and they are strong.
But the knockout rounds of the Champions League are unpredictable. We have seen underdog teams win many times in the past. For example, Liverpool were at the top of the table in the first round last season. But they lost badly against PSG in the first knockout match and were eliminated from the Champions League. Although Arsenal have a better chance of winning, if they fail to perform well, they will not be able to qualify for the quarter-finals.