I have the soft shoulder bag for my one year old boy. He's 11 1/2 lbs. Tell you why I prefer a shoulder bag.
My boy dosen't stand while he's in the bag. He lies down facing the open flap with netting so that he can see the outside while being carried. Flap can be closed if you don't want kitty to be afraid of the outside as some kitties freaked out going out. It is more comfortable for him and also for me because the bag is being carried very close to my body and the bag does not swing and bounce like those short handles type.
Another good thing about it is if your kitty is afraid of sounds and sights, you can put your arm on the bag and hold the bag much closer and tighter and when slung on the shoulders or move the bag to your chest just like carrying him in front of you and that way your kitty is much closer to you and you can talk to him and give him assurance. Those hand held ones you can't do that and the hard case type also can't do that.
It's less tiring carrying this than the short handles ones. Some comes with slings which can be shortened. Mine comes with two shoulder straps and it's a top loader with zipper netting cover so that kitty can look at you and vice versa.
I don't travel with my boy. Carrier is just for visits to the vet or for bringing him around to the neighborhood stores. Had this bag for him since he was 1 1/2 months old.