At level 22 you don't really need either. A normal bag can hold herbs/souldhsards AND normal items so you actually get more bag space that way.
Soul bag - Until you hit endgame and start doing raids or serious PVP you shouldn't need to carry more than a few shards at a time. You can always just make more when you get really low. Unless you routinely carry more than 16 shards with you all the time, a soul bag is just a waste of space.
Herb bag - Do you ever really find yourself with more than 16 separate stacks of herbs in your bags? Especially at lower levels when trips back to town are semi-frequent, that shouldn't ever happen. Even if it does happen though you only lose an extra bag slot temporarily, not permanently.
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75 Rabbit/75 Sheep/75 Coeurl/75 Eft/75 Raptor/75 Hippogryph/75 Puk
75 Scorpion/75 Wamoura/75 Pixie/75 Peiste/64 Sabotender
51 Bird/41 Mandragora/40 Bee/37 Crawler/37 Bat
Items no one cares about: O
Missions no one cares about: O
Crafts no one cares about: O