I'm surprised no one has mentioned the cross-band repeat function of the 7900 that the 7800 doesn't have. Also, not sure if it matters to you or not, but the 7800 has lighted buttons (7900 does not). This was a big selling point for me, since I planned to use the radio in a vehicle. Some would argue that it's not that important once you get used to the controls and learn where all of the buttons are. Something to think about...
Also, I didn't know the 7800 had been discontinued! I just got one back in January. Oh well, still a great radio; I've been very pleased with mine.
*EDIT: I just went to Yaesu's website and apparently I'm out of the loop. In addition to the 7800 being discontinued, it looks like the 7900 got a minor redesign, as it appears to have lighted buttons now. Who knew...