I agree with newtekie1. If you can afford all SSD, go all SSD. Sure, it will cost more initially, but spread those costs over the expected life expectancy of the computer, and the added cost washes out with some cost savings in lower energy consumption and heat generation too (less heat matters to facility air conditioning - pennies, but pennies add up).
Having an SSD as your boot is NOT just about boot times. Sure boot times greatly improve, but how many times a day do you boot your computer? I only boot my computer when some update requires it! So it could be days or even weeks between boots.
I've been running with SSDs for almost 4 years now and I am still amazed at how fast my systems boot or wake from sleep. The bottleneck is me - the systems have to wait for me to enter my password. But the main point is after boot, everything Windows does operates so much faster on SSD. And the faster Windows tasks are completed, the quicker Windows can deal with your programs tasks. And if your programs are on your boot drive (or another SSD) too, they operate much faster too.
I have a shortcut to a Word document in my Quick Launch task bar. This document is 70 pages long with charts, images, links and more. I use this daily as it contains my canned texts and references I use for "working the forums". When I double click that shortcut, Word starts and that document is loaded by the time I move my mouse from the shortcut to the open document. I mean it "pops" open! That cannot happen with conventional hard drives, even with the fastest hybrid drive.
As for M.2 vs SATA III SSDs, no doubt the M.2 will be faster. But we're typically talking milliseconds here. You would be hard pressed to actually notice the difference unless you had two otherwise identical computers side-by-side performing the exact same task.
You do need to look at the type of data you have too. I mean who really has 10TB of data files they regularly access? And what type of data is it? If mostly tunes or video/movie files, they can go on a hard drive. Except for locating the first file segment, fast access and loading times don't matter.
A nice 250-256GB SSD boot drive will easily hold your OS, drivers, and most, if not all of your applications and leave plenty of free space for Windows to operate in, for your Windows managed Page File and all Windows and program temp files.