It's going to be decided in the pack. Should Northampton gain supremacy in the tight five and back row, they'll be in a position to bludgeon Leinster in a similar manner as they did Ulster and Perpignan. Should Leinster achieve parity, they have a huge advantage in the half backs and midfield which they should be able to exploit. While Northampton have a very dangerous back three, Leinster's is in or around the same level and it's all inconsequential if you midfield can't get you the ball.
I'd fancy our pack to stand up. We've taken on Clermont, Toulouse and Leicester, who were all very strong in that area and the boys have held firm. When you look at the teams Northampton have played this year on the other hand, you'd have to say they haven't really been tested by a top team yet. If the boys can keep to their tasks and not surcome to the over confidence on display in this thread, we should be able to take a win. That said, Northampton are a good team and anything can happen in a cup final.