The areas of application where a Hexa can go thanks to its AWD (with a theoretical maximum of 33% power to the front wheels in a heavy RWD car) and the smaller, lighter FWD Harrier can't is small.
Avenue 2 - Straight up, just about any car can do Spiti in the winter - white Spiti is a different matter though. That's when the Hexa's AWD will make a difference (snow cases only). Again though, heavy rear biased with not much of an AWD isn't that much better than a better GC/Wheelbase ratio-ed, lighter FWD when climbing hairpins on mountains. That said, as far as Spiti goes, you can go in winter and see the snow separately (nice and close) without needing to drive on it.
Avenue 3 - Off-roading to campsites needs to be judged on a case to case basis - there'll be places a Harrier would reach, and places where even a G-Wagon or Wrangler wouldn't.
Avenue 1 - The heavy RWD Hexa with such a limited power distribution won't manage dunes. The lighter Harrier won't either, but again, the (lower) weight might even be an advantage in certain situations.
FWIW, I just did Spiti and the Dunes just about back to back (15th Nov to 1st Dec, then 5th Jan to 15th Jan). Saw many cars stuck in the sand - not dunes mind you, sand covered roads stretching across the DNP.
Given what I see of what you've asked, I'd recommend the Harrier for you. IMO the Hexa is a great car, but not an off-roader. If I didn't need a seven seater, I'd pick the Harrier any day. For an expedition, they are equivalent in terms of number of people which can be ferried. The Harrier is newer and lighter and will be nicer to use daily. Most expeditions practically don't
require a 4x4 - you can do without one 99% of the time by being a little cautious, sensible and willing to walk more. That said, nothing like the freedom a proper 4x4 gives you. Which the Hexa won't.
That said, the Hexa is a proven, trouble-free vehicle by now. The Harrier a new one. Also, for Indian conditions, I personally prefer body on frame - especially since you intend to keep the vehicle long term. But then the Hexa is aging (possibly even on its way out if some are to be believed), the Harrier modern.
In you shoes, I'd wait for the Harrier to be in the market for a year, and then pick it up. If you want a more off-road focused vehicle (mostly for the dunes given the parameters so far), you must either increase your budget, or go for a utilitarian off-roader that'll leave you with the risk of multiple disc herniations over the course of an expedition (mild exaggeration).
Oh, and it's absolutely okay to have a seven seater and not use it mostly - the day you need the space, it really comes in handy. Also, sometimes, even just two up, my car is full - with off-road equipment and camera gear etc.
Finally, I think I understand what I've been trying to get at all through this post:
Practically there isn't much of a difference. Both have advantages and disadvantages. Just go for the one
you LIKE more. I would personally pick the modern car.
And remember 90 - 98% of an expedition is highways/sealed tarmac. Beyond that, there'll be places every car has to stop at. Be clear about your priorities.
If you want to true freedom, sacrifice comfort or money.
Also, whatever you go for, buy a shovel and tow-strap before Spiti, and sand ladders before the dunes. Mentally prepare to dig yourself out (real physical rigour may easily be involved), and you can have that freedom I keep harping on about anyway.
P.S. With all the cross shopping activity and chatter, this thread was bound to come up.
I hope this is turned into a poll. Also, I love that it's framed as an expedition vehicle question. Also, clearly the Harrier begs for an AWD (unfortunately economic sense can't be damned by Tata).