The Surface Pro line is like the Rolls Royce of tablets. It's premium in every way. Sure, some of the alternatives are really nice too, but so is a Cadillac or Mercedes. Some people swear up and down that their Mercedes is the nicest car in the world, as they trade their lease in every 2-4 years, whereas many people who own a Royce tend to keep them a long time.
I can't believe Apple released the iPad Pro still using iOS, what a waste! I'm sure it will still sell, but I really don't know why. Over $1000 after buying the pen, and your hard drive (not expandable with MicroSD) is only 32 GB. And it still only runs phone apps! What a fleece job.
The XPS has always been a nice line of hybrids, but never nice enough to get me to buy one. The hinge doesn't seem that special to me and while a 4k device is an amazing feat on such a small screen, I can't imagine it's necessary. The m5 processor probably gets better battery life, but has a weaker graphics processor. The XPS may be a great alternative for people considering the SP4 m3.
I haven't seen the Spectre X2, but the kickstand seems a bit gimmicky, and I wonder how long it will be until it's worn out. Maybe it gets a better battery life, maybe not... nobody can really trust manufacturer figures. Tablet (and laptop) audio has never been that important to me. If I can hear it well, it passes. If I want high(er) quality audio, I'll use some nice headphones.
Lenovo has always made a good laptop, and it seems the Miix 700 is no exception. It's a simple design, which means it's not going to break as easily. It's cheap, which is great, but it has no i processor options, which means worse graphics ability. Based on price, this is probably the best (or only) alternative on the list, IMHO.