Well, if you already have a 7 channel amp, the amp section of the 5008 won't do you much good, unless your 770 goes out or you want more than 7 speaker channels.
If I had to pick, I'd probably take the 5508, so I could get MultEQ XT32 and the Sub EQ, but I have no way of knowing if you would want any of those features.
Seriously, if your going to spend $1500+ on a new receiver or processor, you need to do a little more research on your own or give us more to go on with what you are considering (i.e. why those 3 models, what features you are looking for, etc).