It doesn't replace the Hubble. It does have a mirror that's about the same size as the Hubble mirror. The Hubble's time is very heavily subscribed, and it will be updated in a 2008 shuttle flight, and another space telescope will be launched after that. The SOFIA aircraft will have much more time available for infrared astronmy and can be flown anwhere in the world which comes in handy if an astromical event is only visible from the southern hemisphere. Due to the fact that the newest SOFIA platform is a 747, it has lots of room for scientists and equipment compared to previous platforms like the C-141A prototype plane.
I presume that it's also able to get to another location far more quickly than the hubble! :D
The most important difference is that it has a primary mirror 3 times the size of the Kuiper Airborne Observatory.