Hey Coke, sorry if I'm too late, I hardly ever visit the board anymore... I've never done single rider, but I'm pretty sure you miss the dungeon where you pass by the one eyed witch statue and the door to the potions classroom (where you hear a professor chastising Neville if you listen close enough), you will also miss the greenhouse (where you see mandrakes, which is really the only cool thing about the greenhouse otherwise its just a long hot wait), but the best part is when you enter the castle, you pass the hourglasses for the house contest, a statue of who I think is Godric Gryffindor, and Salazar Slytherin. You might go through the Headmaster's chambers where Dumbledore welcomes you to Hogwarts and talks about a few ride plot points I shant spoil. I believe the single rider line does take you into the DADA classroom where our intrepid trio talk to you about skipping Binns lecture. I think you miss the Gryffindor common room and the Fat Lady's portrait, but you do get to see the sorting hat just before boarding.
As a huge Potterhead, I'd rather wait with a good friend than do single rider. Hope that helps!