You could also save your points until next week and give Shaco a try which will let you give him a testdrive and see if you will want to buy him (even if not immediately, for note later)
As for who to buy, I would get whose playstyle intrigues you the most.
Pantheon excels in small enounters and relies heavily on the large single target and aoe damage from heartseeker strike as a burst on people. Due to being melee he is far more vulnerable in 5v5 encounters and generally doesn't scale well into endgame.
Katarina can be built as attack damage or spellpower, but either way you build her, she plays similar to a caster. You dont do very much auto attacking. She is a fan favorite and her abilities cost nothing, they are only restricted by cooldown. She is a phenomenal laner and is strong from the start to the finish of the game. Kats downside is that later on she is highly reliant on having a good ultimate to be useful, and can run into problems if it get interrupted.