Not sure how helpful my view is as I’ve not used a Niche, BUT I found myself in exactly the same position as you recently - trying to decide between a Niche and an Atom 75E. I chose the Atom. My reasons for doing so were:
-Price: I managed to get a 2nd hand 75E in extremely good condition at £400, which I thought was a bit of a bargain.
-Looks: I’m one of the few who doesn’t like the look of the Niche.
-Quality/sturdiness: I expect both to have a very good grind but the 75E is reassuringly heavy at 9.5kg, made very solidly with lots of metal, and from a manufacturer that’s been around for a while. The big burrs cut through 18g in 2.55 seconds for me at the mo!
Availability wasn’t a problem for me as Niche had very quick availability at the time.
Since buying the Atom I’ve found what for me is a very minor niggle, that I need to adjust the grind size ever so slightly as the hopper depletes, but I expect to do that as the beans age anyway. I purge about 4g once per day which I’m fine with - those are the only two negatives to it really and I’ve been extremely happy with it. I’m not getting bored with the insane grinding speed anytime soon, and the grinds have an excellent consistency, no clumping, and produce a very good shot with no WDT (although WDT does improve it a little).
I can’t profess to saying the 75E is better than the Niche, or that the 75E is the best in it’s price range for a big flat burr, but I can tell you I’m very happy with my 75E. Hope that helps.