I have used both, although my last use of 777 was more than a year ago.
Main difference is that 777 is one of the thinnest canvases out there, while Lyve is fairly thick. And I think 777 has about the finest weave of any canvas I have seen, although the very latest batches of Lyve apparently are also fairly fine however I haven't seen any up to date samples. The thicker Lyve canvas might be a better choice if you plan to stretch prints more than about 24" on the vertical, thicker canvases are somewhat more sag resistant. I always mount canvas, so I can't say too much about stretching. As canvases go Lyve is quite stiff whereas 777 is quite flexible. I would suggest making a few test wraps before committing to either one. I have a few canvas stretching buddies who still seem to prefer the old BC Chromata canvas. With custom profiles Lyve or Chromata will probably give you a wider gamut than 777, but the difference would not be signficant for most prints. Looking at the printing profiles I have, Lyve's gamut volume is a few percent bigger in the blue and green direction.
For a long time 777 was my absolute favorite canvas because of the fine weave. There were some issues with edge roll which caused me to give it up, I think those have been corrected.