I chose the 880 for personal reasons that don't really matter to anyone but me.
I will say this. I'm 37, loved car and home audio for 20 years, and Pioneer has always been able to prove a definate upgrade between models whether you can find it in the spec sheet or not. Even their home receivers will use better components in the higher end models even though some of the features are the same throughout the line.
That being said, none of it matters if you're playing MP3's and driving 60MPH.
With all that noise (and it's there even if you have 20 layers of dynamat) and the MP3 compression you won't be able to tell the difference between the 780, the 880, or a Jensen from Walmart.