The DD-5 punches through the mix more, especially for dotted eights. I use mine exclusively for that. If you're looking for that U2/Hillsong tone, the DD5 is better than the DD7 IMO. If you're going for the biggest bang for your buck, it's the DD7.
In terms of sound? The DD-5 is brighter, the DD-7 has more tonal options: tape, analog, etc.
I prefer the DD-5 for clean sounds, it's VERY crisp and present. It doesn't try to sound "analog" and that's its strength. The DD-7 is engineered to be more of an all-rounder.