Kat: Much stronger lane presence until level 6, in which case her ult is hit or miss against her enemy.
Akali: Moderate lane presence before level 6, when she dings, she will kill someone in her lane if played right, snowballing from there.
Both if fed wreck teams, both jump into the fight late, Akali is a better chaser, but Kat can pull out easier and has better harass...
Akali's more "I burst your face" while Kat is "I'll work you down till I can kill you"
Pick one that fits your playstyle, all I can tell yeah.