Well, I haven't ridden one, let alone seen one in the wild. That said, it is an intriguing bike, though I'm still not sold on discs for road for most people. Lest you call me a Luddite, I'm quite the technophile, and have discs on my three MTBs.
If you haven't seen one yet for sale, your chances may dwindle quickly. Being the top shelf model, they don't make many of those in the first place, so folks who want them snatch them up early. I was after the original Solace Premium a couple of years ago with rim brakes, and they sold out quickly.
Comparison wise, the frame should be darn near identical between the Prem, 20, and 15, though the Prem uses HMX carbon. As you're no doubt aware, the Prem also has super nice carbon wheels, plus Di2.
A friend has had Di2 for several years and still swears by it. I'd love to try it, but again, like road discs, I'm not sure it'd be worth for me.
Check the dealer locator and start making phone calls for availability. You might even try Scott customer service, as they can look to see what dealers booked them, and might still have one.
I had a great experience dealing with and purchasing from Salt Cycles in Salt Lake City, as they move quite a bit of Scott product.
Good hunting, though. If you find one and get to try it, let us all know, since over 100 folks have looked at this thread.