I play a lot of bladelocks, over half the 5e characters I've made, and I've love the SCAG cantrips. With this in mind, may I suggest... maybe neither?

Booming blade is good damage that scales well, and it helpful because it's a penalty for targets moving from your melee reach, but it's just a penalty; it doesn't stop them at all. Green-flame blade does good damage and scales well, but the damage is fire (easily resisted) and it's only one other creature.

Both are nice because they add to your weapon damage, but I like booming blade much better than green-flame because it's 1) an effect you don't get elsewhere vs more damage, 2) thunder is better than fire as a damage type, and 3) it's a damage double dip rather than spreading it out.

Lightning lure on the other hand does good damage, and is one of a handful of effects that forces movement toward you - which is where a bladelock wants their enemies. I've found lightning lure to be very useful for bladelocks (and eldritch knights or anyone with the sentinel feat), more so than either attacking cantrip, but that is subjective.

With the warcaster feat, green-flame blade falls well short of the other two. Great, more damage, woo-freaking-hoo. However, booming blade does double damage as the target provokes AoO (although disengage does negate it somewhat), and lightning lure stops the AoO provoking movement.

And the fourth cantrip....I also think that sword burst is better for a bladelock than eldritch blast (yeah, damage that's great if you take some invocations, blah blah blah). Blast doesn't work in melee range, which is where you're going to spend most of your combats, and sword burst is force damage too, negating one of the touted advantages of EB.

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