the hors piste guide does have a fair bit of overlap with 7+8, but also includes loads of the best 6's which is nice, plus loads of stuff not in 7+8, in general bit more generous on the grade side, has lots of good photos, and it's available, which as far as I'm aware 7+8 isn't.
If you need a guide now, get it (decathalon at font had it in stock as of two weeks ago).
If you can wait, I'd hang on for the update on 7+8 (but we all know about guidebook publication dates...)