The waist strap on the Huskys is a huge plus in my opinion. The Husky 580 is definitely a good bit more powerfull than the 770. I have run them side by side. I do agree with Ridin' Green that the 580 doesn't scour all that well from the factory, at least for a tall guy like me. I was disappointed in that part when I got mine, having used Shindaiwa EB630s with a long tube that scoured well and I could get right down to the ground. Yes it will scour stuck debris, but it takes longer than it should for such a beast of a blower. I added an extra intermediate tube to mine, and with that on it scours exceptionally well. I was going to do the same had I bought the 770 after running it at the dealer and being surprised that it didn't scour that well either...I'm a pretty tall guy putting the tube on both of them a good ways above the ground.
I posted this video elsewhere, but here it is again. I think the 10 minute mark on shows best how powerfull it is...look at that dirt fly!
I haven't run the 570, but I think Ridin' Greens recommendation for the 570 is probably a good one for most operators as an all-around blower. I think for my purposes, which includes blowing literally miles of dirt road and trails, the 580 was the best blower for me...that massive amount of air flow is good for blowing branches and light brush.