Ez main here, AP will give you much more damage overall and a 100-0 burst on most champions. With 40% CD you can spam his Mystic shot every 1 second as long as it hits something. Imo lichbane is better than Runeglaive currently if you were going to try and 100-0 other champions, and when you run runeglaive you are forced to take smite. AD ez sit around 15-25% cooldown reduction but W becomes useless and I wish they would change it. I don't even put a point into it until I have to. (38-9 ranked record with Ez including troll/afk games)
Depends on the match you are playing and the champions you are playing against, however, if you want to just have fun and pubstomp a normal game, AP is by far the way to go.
If you want to play ranked, AD is the way to go, however he only fits into Poke comps really, as his bot laning is extremely bad and if the other bot players are good, you are not likely to hit many mystic shots (his main damage since his autos are the weakest of any adc)
Pros of AP
-fun to play
-ultimate becomes a nuke, if you manage to hit all 5 in a teamfight, you automatically win
-Higher Mobility 40% cd 3-4 second arcane shifts for escape (while hitting mystic shots)
-100-0 burst (you can do this every 4 sec like Leblanc)
-W goes from unusable to a nuke
-E also becomes a nuke
-better penetration options
-able to buy zhonyas
-ult will clear an entire wave instead of leaving minions at 20% hp
Cons of AP
-auto attacks do nothing (extremely difficult to farm)
-passive is absolutely useless
-No lifesteal (can spell vamp but that requires detour of build)
-skill reliant
-terrible farming and waveclear even at late game (unless you ult) if W could hit minions as well that would be great (plz riot)
-banshees veil ruins you