First question for me would be square crown vs. round. Round is nice if you are doing a lot of seat out tricks and have a tendency to hit your knees (hard to know without having done much trials, I know…). If you can see yourself ever doing anything that requires a foot on the frame (like 1ft riding), I’d go towards a square frame, most of the newer “rounded off square” frames are pretty slimline and don’t cause much issue for most of the riders.
Athmos is a decent uni - it should be strong enough but a bit on the heavy side. (I’ve never understood peoples issues with stamped bearing housings btw…). It should however be noted that while it’s strong, it’s also very much “low end” in every single component except for the wheelset and saddle. Seatpost, tire, cranks even pedals.
If this was going to become your “main” uni and trials/street/flat your main focus, I’d probably recommend spending a bit more money.
Kris Holm, Impact Gravity, Mad4one. All a similar level in my eyes, as I said I’d decide on round crown vs. square crown first, which might rule out the Gravity. I’m not a particular fan of KH spirits, but if you don’t want to do many flatland roll tricks where you stand on the cranks, they are okay. Mad4one depending on powersquare or Mad-Tecno might be slightly more expensive or the slightly less money, I’d personally choose them over KH though.
I’d throw another thing in the mix, if it’s also something you might do some riding tricks on, I’d consider one of the wide tired 20" (Sylph or Mad4one). I think I’m a lot more sensitive than most in this regard, but I really don’t like riding knobby tires on asphalt/concrete. The 19" trials tires do have a lot nicer “bounce” though, so if hopping really is your main focus, they will probably be your choice.
I think @elpuebloUNIdo has a fair point and sometimes honestly I’m a bit guilty of getting too caught in thinking along those lines. But you also can’t fully ignore the “value for money” perspective: Is paying double the money for a M41 going to make my riding twice as fun as on an athmos? I’d personally doubt that. Like with pretty much all equipment purchases, there are diminishing returns after a certain point for unicycles and especially for people new to the sport, I think it may be hard to realize where those start (not that that is something you can clearly define). But if you have the money to spend, there is nothing wrong with going to the 100% best option first.