It is better to trust Brighton with the winning than giving it to Manchester United. Manchester United didn't conceive against Fulham and Brighton didn't conceive either against Everton. This is an indication that both teams are good defensively, judging by their games so far, so it would be best to play the bet under 2.5 goals. That's a more likely outcome, since both teams are boasting of a solid defense.
Manchester United are disappointing but I can neither go for Brighton. About under 2.5 goals bet, I do not think it is good to use just a single match to make such conclusion. At least, you should look for 3 to 5 matches between both clubs but the season has just started. I am not implying that you may not be right but a single match is not enough to make such conclusion. But if I still remember what happened in last season, it was hard to score more goals while playing with Manchester United.