Gangplank or irelia

Irelia is generally stronger. Actual mobility, actual CC, free tenacity. GP scales a bit better I think, in terms of pure damage... Plus with the proper build he gets tons of MS

GP is getting reworked, so you might buy him now to try his current gameplay before it becomes history, then try the new one. Two champs at the cost of one u.u

It's all based on your enemy's champion selection, but I think Irelia has more sustainability than GP.

Irelia can build tank and damage which she can survives longer in team fights, but GP can only deal damage and he's squishy too, basically when melee champ comes up to your face you are dead in no matter of time.