I can compare.
I had the original SX Pro (daughter lost it literally the day before I got the NS-Atmosphere, that's either great or rotten timing). I see there's been a small hardware revision since mine, looks like the usb-c is a bit longer (definitely noticeable on pictures of the SX Gear). I had to purchase a usb-c extender because the original SX Pro would not work with the Nyko case on the Switch. I can confirm this is not an issue with the NS-Atmosphere, it's raised just enough to clear the case. The size of the SX Pro plus usb-c extender is comparable to the size of the NS-Atmosphere. Also, the jig from the NS-Atmosphere seems to fit a tiny bit better. Slides right into place, whereas the SX jig would slide mostly into place, but needed a bit of a push to ensure proper contact. I love that the jig fits into the NS-Atmosphere, though it's a tiny bit too snug. I kept the SX Pro, a usb-c extender, and a jig in a small plastic bag, it actually took up (barely) more space than the NS-Atmosphere.
As for actual use, I prefer the SX Pro. There's been a time or two that it let me down, didn't have enough juice to send a payload. After I figured out what was up, it was simple enough to fix (either plug directly into a usb-c charger, or get a micro usb cable to charge for a few seconds). I also like that it's ready to send payload after payload. Was dead simple to explain how to use it to my wife and kids. If the Switch crashes and has to be powered off, plug in the dongle. Turn on Switch. Simple instructions, always worked. Not so with the NS-Atmosphere. See, sometimes (but not always, oddly enough) when you do a poweroff on the Switch with AutoRCM enabled, it'll go back into RCM mode. Send a payload when it's in this state and it'll actually poweroff. So the instructions are now "Turn the Switch off. Turn the dongle on. Plug dongle into the Switch. Turn on the Switch. If nothing happens, press the RST button on the dongle and/or turn the dongle off and back on." That's a bit more complicated than "Turn the Switch off, plug in dongle, turn Switch on". Plus they have to remember to turn off the dongle, whereas before they simply had to wait until it was fully booted to remove (this ensures at least a few seconds of recharge time for the SX Pro).
I don't know what's on the NS-Atmosphere from the factory, when I plugged it in the Switch and booted, it just went to a black (but backlit) screen. I flashed the SX payload so it acts just like the SX Pro, looks for boot.dat on the microsd card. I like how it can be changed, but wish there was a way to flash multiple payloads and select without connecting to PC (I know this is an option on others, just saying). The SX Pro is more user-friendly in this regard, no need to install drivers (fyi I didn't have to install any, it just worked on my Windows 10 laptop), flash (note, you must close the app and reopen to flash another dongle else you get an error), etc.
That said, the NS-Atmosphere is dirt cheap, I got five for $51.45USD. If you don't want SX OS I think the NS-Atmosphere is a no-brainer. If you DO want SX OS and can wait for delivery, I'd consider the SX Pro vs the NS-Atmosphere plus separate SX OS license.
