What one is better at the role ap?

Sion has a nice stun and with ult he can stay in lane for awhile and farm because of the lifesteal. Also he get free hp from his E and a awesome shield that does crazy dmg.

Kass, I dont know whats that good about him besides his ult. But my friend insisted that he is so much better.

Kass is the stronger choice. You're right in that assuming his ult is his main strong point, because mobility on this map is really important. He can Riftwalk over every wall on the map. He also counters squishy AP's really hard, as his silence makes it nearly impossible for them to retaliate when he harasses them.

AP Sion just doesn't bring enough to the table, especially late game. He stuns and shields, then dies.

I would agree , Sion is much better early game, but since early game is pretty short on this map, Kassadin will be better the majority of the time.

Sion is stronger then kass early game. But like what yetti said kass can rift over any wall. Overall kass would be better due to sion being countered by bansheeveil(half sion dmg gone).