I used to use a hard carrier for the vet trips and the soft carrier for emergencies (we had a fire alarm) but will use only the soft carrier in future. It's practical, easy to carry the cat in, and can be used on a plane if necessary. I would only want the cat in hand baggage, never in the hold.
If you want to 'lock your cat in' a hard carrier is more advisable since it has a spring-lock system that a cat cannot open. Zippered soft carriers are slightly more difficult to close, but the cat (unless it is unnaturally prehensile) won't get that open either.
If you want to 'lock your cat in' a hard carrier is more advisable since it has a spring-lock system that a cat cannot open. Zippered soft carriers are slightly more difficult to close, but the cat (unless it is unnaturally prehensile) won't get that open either.