I own both. I will be perfectly honest, if I didn't consistently need more than 4 channels, I would stick with the R-4. I think it is designed better and the 80 gig ard drive is wonderful. Recording 8 channels @ 24 96 at the last gig I had, I could not get 2 sets on the same 16 gig card, had to pull and dump after every set. Really missed the HD. My R4 is modded. I think the sound is better on it than the 680 ut the trade off for the extra channells is worth it for me.
the 680 is a kick ass recorder if you are going to be using more than 4 channels a lot. If I weren't, I would have stayed with the R-4 personally. If 6 or 8 channels is going to e the excption, not the norm, I don't think the trade off in quality, intuitive functions,sound, and reliability is worth it to only use the extra channels on rare occasions