Someone else > Katarina > Pantheon
Katarina is more viable, burstier, and generally kicks more ass than panth imo with her Kill Shunpo --> Death lotus ---> chase with BB and Shunpo for the kill.
Pantheon on the other hand, once his HSS runs its course, its very easy to run frmo him if you have any escape mechanism whatsoever. Also, Kat's better at diving people Like Yi or Veigar, because they try to fight while DL cuts them down, They die, Kat Killer shunpo's away while rofling.
But They both fall pretty easily.
Amumu is a HARD counter to Kat with a double stun, completly nullifying her ult while beating her down through sheer tankiness (made up words FTW). Another good counter to Kat would be the omnipotent Jax, who if he gets a counter strike simply Rapes Katarina into next week by relentless assaulting her, then expower --> Leap strike when she shunpo's away in order to close it up and get the kill.
anyway, other than people that can kill her, There are also some better contributions to the team then Either Katarina Or Pantheon. In the 3150's bracket, Olaf, Shaco or Garen are somewhat better choices. Olaf and garen are bulkier, deal close to katarina's damage, (Judgement after the buff deals almost as much damage as DL now..fml) And who doesnt run from a 300 pound beserker running at you axe flying, lightning ripping through the sky to cleave your flesh open? Shaco is an iffy choice. He's relatively squishy, but the stealth and JitB help your team a lot though scouting, warding, and generally being a ***** in team fights. Just my thoughts