I will only use hard-sided carriers for the reasons mentioned above. Also, since a quality carrier should last through many pets’ lifetimes (rabbits, cats, small dogs, etc…) I like knowing that they can be fully disinfected.
The first carrier I had was a small one without a top door. When we adopted Max, I needed a larger carrier so they would both fit for vet visits. I found a great, sturdy, medium Marchioro brand carrier at PetSmart WITH a top door. Now, I won’t use a carrier without it! A top door makes all the difference in the world.
Soft-sided carriers can’t be cleaned easily, they are susceptible to being chewed or soiled, and they offer no protection in the event of a car accident or against other animals (like a curious dog in the vet’s office waiting room). I can also imagine that ventilation and air flow through the mesh isn’t going to be as good as the openings in a hard carrier.