I like this video explanation of the difference between the hard shell seat and the mesh seat for trikes. However, I do have few points to mention which you find under the video.
Hard shell seats
– They usually make the center of gravity of the trike lower so the trike is more stable while riding through curves. But they also usually require you to lean more as they have limited side support, especially in their upper part.
– The Ventisit seat pad is sliding so may have a feeling of getting out of the trike while cornering fast. However, Ventisit offers their seat pads with some kind of non-sliding treatment.
– They are breathable which is fine while riding in the Summer and during hot days, but is pretty bad when riding in cold temperatures. You feel the cold on your back then. I recommend adding some additional layer in between the two layers on the mesh. Something like a camping mattress. AZUB even offers it as an accessory for their trikes.
– Some of the mesh seats have sizes as well nowadays. Just ask your dealer or manufacturer.
– There are also some extra comfortable highly padded mesh seats available nowadays. Those are not so breathable, but look great and are very comfortable. Offered by ICE, HP Velotechnik, Catrike and some others as well.