A few ‘basics’ then if you’ve not had anything new since 1998.
In 2020 (or 21) nearly all will be ‘1x’ ie no front mech and 1 rong up front And 11 or 12 out back with a big range. (Let’s not grt into whether that is a good thing… but it is THE thing).
Hydraulic disc brakes at virtually any price point.
Two wheel sizes. 27.5 (“650B”) or 29"
The traditional 26 is near dead (certainly on all new bikes for 4 or 5 years).
Most folks run tubeless. Just as with road bikes, be aware most are set up as tubed from the shop.
As someone 6ft 3" either wheel size works for you. 29 roll a bit better. But not quite a nimble when it gets tight and twitchy.
(For shorties, 29 can sometimes be a bit unwieldy / the size of 2 wheels can sometimes (not always tho) compromise the geometry.
Geometry… long / low / slack ! It does make the bike WAAAY more stable and you don’t feel like you’re always an inch from going over the bars.
Tyres ? Wider ! My 2.35" are almost seen as old stool skinny in some places. 2.4 to 2.6 is quite common.
Absolutely NO bar-ends !!!
Dropper seat posts. They really are worth their weight in gold if you can get one on / with the bike (telescopic seat post that goes up / down like an office chair, with usually a lever to actuate on the bars.).
Best 2 things ever in mtb ? 1. Disc brakes. 2. Dropper posts.
As far as the full suss vs hartail.
I have both. What I would say is that at your budget, a full susser will probably have some compromises on it in terms of fork / suspension. Or longevity. Cheaper (less stupidly expensive) can need more maintenance and then sometimes unavailability in parts when you get to things like suspension internals.
Depends also what the weather is like where you are. Dry Arizona ? Not a problem. Piss wet through Britain (or BC or Oregon etc) then that tends to find out the weaknesses in cheaper suspension (and especially pivots).
Also whilst some stuff is lighter, the mtb world has mainly stopped obsessing with weight-at-all-costs and values function and good ride character over flimsy noodles these days. (Tho the $10k XC race bikes are ludicrously light !!)
Me ? As 1 or 2 have already said, At the $1500 mark I’d be getting a hardtail with a reasonable amount of travel on the front fork. Because sadly whilst its a lot of money still, you’ll most likely get a heavy pogo stick that needs too much maintenance.
I’d defo rather have a hardtail with a dropper post than a full susser without one.
At $2k there just about starts to be 1 or 2 full sussers…but not the ‘big brands’ like Spesh or Trek.
Take a look at this article on Singletrack for some calibration (yep British-centric as it’s British !! But gives you some ideas still about where it’s at).
https://singletrackworld.com/2020/10/best-bike-under-2k-singletrack-reader-awards-2020/
For info, Sonder are a UK based Co that are sort of direct-sell (tho they have a handful of real shops too - they are part of a British outdoor gear Co called Alpkit.
Hope this is of some help.
And Enjoy the ride !!