I think there is at least one special case here, that being AP
. I did a small bit of calculation assuming Ezreal is level 9 and has maxed Q and has 2 points in W, with a small poke combo of Q + W. I didn't account for AP from runes and masteries, but if I did, it would just make the difference between Lich Bane's damage and Morello's greater, because while they'd get the same amount of proportional damage increase from that added AP, Lich Bane benefits more from it due to the Spellblade passive scaling with AP as well. I didn't see the need to calculate that since Lich Bane already outdamages Morello in a small burst scenario. If you gave enough time, only using these two items, Morello would eventually reach a higher DPS since it has a higher amount of CDR given, though I believe it would take a good bit of time for that to occur, because the sheer amount of dmg that Lich Bane's Spellblade passive gives is so massive.
:
Q = 115 + 1.1(71.9) + .4(100) = 119.09
W = 115 + .8(100) = 195
Q + W = 314.09
:
Q = 115 + 1.1(71.9) + .4(80) = 111.09
W = 115 + .8(80) = 179
Spellblade = .75(71.9) + .5(80) = 93.925
Q + W + Spellblade = 384.015
After doing this, I think it is still more beneficial to get Lich Bane first on
as well as any champions that can easily apply Lich Bane and excel in smaller trades and poke combos, like a
or something else similar, since they flat out get more damage from the item when used correctly, it would be better to build Morello after Lich if you're just talking about dmg efficienty. Meanwhile, DPS mages that cast frequently and often don't include auto attacks in combos would probably benefit more from Morello first.
tl;dr It's still a debatable thing, really, just pay attention to who you're playing and try to identify your strengths/trading style.
PS: I didn't think this would format as large as it did, my bad for not using the preview function to make this a little easier on the eyes...