As Katarina, you will mostly be playing AP (slightly hybrid) but you won't have high attack speed or any crit. You'll be using your skills to nuke. With Fiora, you've got a steroid which boosts your attack speed and an ultimate that you can abuse for invincibility + on-hit procs (bloodthirsters will get you to full health; stacked tiamats will ace a team if they're grouped together).
Neither of them are really stealthy, but katarina teleports to her enemy instead of dashing to it. You also get placed behind the enemy which is great for your ultimate if they try to run behind you to get away (towards their turret). You need to combine Kat's skills and mind her cooldowns. When all your skills are on cooldown, you're useless. Fiora has a lot of right clicking power. I like Katarina a lot personally.