Keep in mind that you can play both and some matchups definitely favor Akali over Katarina. For example, Akali has sustain and multiple gap closing opportunities and no channeling ability so she does better (if not necessarily well) against Kassadin. Akali has more windows of opportunity so champions that have smaller windows of opportunity for engaging (think Morgana or a team with Janna) will fair worse against Akali than Katarina.
It's also worth noting that Katarina is an AoE mage more than she is an assassin and her damage comes over time because they moved a lot of damage to her ultimate. As a result, she's more susceptible to CC (think Taric) than other champs. That said, Katarina is a much better team fight champion; Akali will have trouble with peel, especially in tank meta. Katarina also does better in laning phase than Akali since she's not resource limited (for the few times it does matter for Akali) and she has better poke and harass. Both snowball really well but Katarina has a better time translating that to a win just because of how well she does in teamfights while Akali does more work to get rid of 1 or 2 priority targets but can't win entire teamfights. On the other hand, Akali has good sustain and vision from her W which has its own niche set of uses.
It's your choice but don't think you're limited to 1 champ.