I’ve got a shan and am delighted with it. If the trail is pointing downwards then its just incredible, it would utterly destroy previous hardtails that I’ve owned. There’s a review I read in Dirt that convinced me to get it, not sure if its available online or not.
Google images privee shan 917 for some much better images of it.
Having said all this, the slackline is very highly regarded. I’ve not tried one, but I don’t think you’d be disappointed with either really. Again to reference the Dirt article, they said both the Shan and Slackline were the 2 best DH orientated hardtails you could get