Somewhere in between would be my guess for a few reasons:
-It's got a negative reputation compared to the 360, after Microsoft tried to end game ownership
-It's coming out at the same time as PS4, and is less powerful-360 had a year's head start.
-These consoles are less ambitious than the last ones were. 360 was pretty cutting edge at the time, roughly matching high end PC specs. This time they're...certainlly not BAD, but especially the One is well below a pretty normal gaming PC now. I'd consider the PS4's GPU high end, though there are several multiple GPUs better, and I'd consider the One's GPU mid range. In other words, it won't be so hard to manufacturer.
-they've also had more time on this-presumably they will have had more time to build units then last time.
All that combined my assumption is it'll be easier than last time to buy both systems, though I'd still expect them to sell out, be a bit patchy until maybe later next year.
Hell, I guess it's a great thing if Microsoft and Sony HAVE built a ton, but still sell out. Means there's maybe more demand than people thought.
I've got one of both preordered. I really, REALLY want to stop having multiple systems, as I just don't have room for them, particularly storing multiples of things down the road. As such, I plan on getting my PS4, seeing if the controller still works even if its battery is removed, the way PS3 does. If not, I may sell PS4, and just switch to One exclusively (though it's a bit painful for me as PS4 is so much more powerful, and I'll always be going "ugh, this would look better on PS4"). I like that One uses non-sealed, non-proprietary batteries though.
My Wii U I haven't even played yet, and while I love Nintendo's games, I'm SICK of this ridiculous multiformat nonsense, and I'm sick of Nintendo releasing last gen systems. If they weren't going to release a real system, they should have sold their games on PS3/360/PS4/One. They'd make more money, and not have to have spent billions releasing an idiotic new system.
*sigh* I hope they go multiplatform soon, but until then I may keep my Wii U and just rent exclusives. I just can not justify keeping three systems and spares, plus portables, etc. in the basement. I'm waaaay out of room, and as it is wish I had spare stuff for my Saturn and SNES and NES.