The Whatsminer machines' squat shape gives the first impression of being a solid little machine, and after running them for a few months you'll realize it is a fact.
The firmware of the Whatsminer is superior, in my opinion, to the Antminer S19s in one significant way. If you're running these machines without air conditioning, the Whatsminer will slow down if the air is too hot, whereas the Antminers will shut down and wait for someone to come along and reset them. It is just a bit of smarts, but it is my favorite feature of the Whatsminer machines.
The fans on the Whatminer M30/M31 series is another great design choice. Eventually you're going to encounter a fan failure. Usually they keep spinning, but the sensor that tells the board how fast they're spinning will give up. When this happens, you'll have to swap the fan out. The Whatsminer fans can be replaced without opening up the miner, making this periodic task far simpler than on the Antminer S19/S17 machines.
Opening these units to clean out dust/moths is a bit more involved than the Antminer units. However in my opinion the other operational benefits outweigh the few extra steps needed to pull the hash boards out.
You won’t get as much hashrate out of the M31S+ as the S19 series and they use up more power per terahash, but overall, these rigs are still a great addition to any fleet given their durability, design, and firmware features.