I'm a pretty big fan of the HP Spectre x360 15. I wrote a review of it and gave it pretty high marks, and I've had occasion to use the previous model for a few months. I'm waiting to get my hands on the updated version (8th-gen Intel Core, MX150) to see how it impacts battery life.
Before the SB2 15" was announced, I was pretty set on the Spectre as my 2017 notebook purchase (will benefit this year from the tax write-off). Now, I'm torn. Some of my own considerations:
1. Display: The Spectre has true 4K UHD, making it my favorite for watching Netflix 4K video. I usually watch stuff late at night in bed after the wife is asleep, and putting the Spectre into media mode with headphones is a great experience. On the other hand, the SB2 15" has the nice 3:2 aspect ratio that's actually better for working. As an investment, that makes more sense even if it won't be able to display true 4K video. Also, while the Spectre has a very nice display in terms of contrast and brightness, it's nowhere near even the SB 13. If the SB2 has an even higher contrast and brighter display, then that's a huge plus in its favor.
2. GPU: That's a huge one for me. My wife's work as a designer would benefit tremendously from the GTX 1060 in the SB2 15". So it would be a machine that we both could make use of. The MX150, by my own testing, slots in almost exactly between the 940MX in the previous Spectre (and oldest SB) and the GTX 1050. Obviously, a GTX 1060 is significantly more powerful, and while that doesn't matter for what I do, it matters a lot for my wife.
3. Battery life: The Spectre gets decent battery life, but at 79 watt-hours with that 4K display, it's not great. The SB2 15" would offer significantly better battery life.
So, yes, the Spectre 16GB/512GB comes in at around $1550, and the SB2 15" 16GB/512GB is (I think) going to be around $2800, there are some really compelling reasons for me to spend the extra money (and to save a bit of it thanks to the tax write-off).
However, if my wife wouldn't benefit from the GTX 1060 in the SB2 15", though, I'd probably end up with the Spectre. It really is a very nice machine.
Note that the Lenovo Yoga 720 has the Core i7-7700HQ (still faster than the 8th-gen Core i7-8550U) and GTX 1050, and so it's another nice alternative. Only problem with that machine is the 70 watt-hour battery that would get killed if you get it with 4K -- and you'd really want to get it with 4K.
I am writing this comment on my Spectre X360 15 that I purchased earlier this year. It is a gorgeous machine, and the best laptop I have ever owned. I do love the 4K display, and I use the Pen input every single day for a variety of note taking, diagramming, and photo editing. The touch pad of the Spectre is Synaptics, and as such does not have the feel of the Precision touch pad on the SB (I tried installing the Lenovo Precision driver, and that does work after a fashion, but it was a complicated hack...)
I am very attracted to the 3:2 aspect ratio and better color accuracy, as well as better brightness on the display of the SB. However, for the price difference, no one can fault the purchase of the Spectre. If I were a gamer or video editor, the decision would go to the SB again.
It is wonderful to have so many options!