Zed can't do what Talon can do and Talon can't do what Zed can do. Simple as that. Talon, when ahead, can walk into a team fight and nuke the enemy teams HP, while Zed can pick people off safely, which Talon can do too, however Zed's more in a splitpushing position where if someone tries to stop him they die because Zed is so good in those situations, Talon has to be more cautious about where he is because he can't escape like Zed can when splitpushing. Champions are good at different playstyles and while they have similarities that does not make one better or worse than another just because they both do "AoE" damage, but imo Zed has VERY limited AoE damage and focuses all his damage on a single target rather than multiple.