Their games are usually filled with a lot of goals. There is even a chance this game might end 2-2 or 3-3, so I thing the over 3.5 option is a nice one considering how big the odd is. But we have to consider that Manchester United doesn't concede much goals now, they conceded more than 2 goals last against Liverpool and against a very good defense, they find it difficult to score. The defense of Manchester United has improved but their midfield and attack is not so good.
Tottenham on the other hand dint mind conceding goals, they keep a very huge line and they know how to score too. This makes they favourites for this game but I can't bet on them. I think I'll follow your suggestion and go with over 3.5 goals.
Manchester United and Tottenham are currently tied on seven points, but Tottenham have more goals than Manchester United. So far Tottenham has nine goals and conceded five, but Manchester United has five goals and conceded five and thus, Tottenham is still better than Manchester United but with such results it will also certainly be difficult to determine the winning team in this match.
About over 3.5 goals it is still possible, but the concern is if they can play with a good defense, then I personally still doubt that the goal option. Because in some moments they also get deadlocked quite often, so they are also unable to make many goals. Thus, it seems like it would be better for me to choose Tottenham in this match, even if only with the asian handicap option or other option too.