Hopefully Barca beat Dortmund and qualify, that's the best case scenario for us.
The last thing you want is for a fully motivated Barcelona with Messi, playing for CL survival in the last game of the group against us. If they beat Dortmund in their next game, they already qualify and top the group, so hopefully they won't be as motivated when they play us at the Meazza. Also they are barely hanging on at the top of the Liga table, with Real Madrid level on points with them, and loads of teams close behind, so it's likely if they beat Dortmund, they will want to rest some of the big players and use them in the league as they will have topped the group and put their focus where it matters.
All of this is a big IF though, because we need to beat Sparta away, and we couldn't even do that in our own back yard.