Arc Trail vs Burst Lightning vs Searing Blaze

Currently I am using 4x Arc Trail, 2x Searing Blaze (+2x Searing Blaze sideboard), 0x Burst Lightning.

Arc Trail does a pretty good job of clearing out multiple guys against decks like Vampires, Caw Blade, Kuldotha Red, and other swarmy-type decks.  It gives the potential for card advantage, to go along with Staggershock in the main-deck.  It can also kill off planeswalkers (2 to your Jace B., 1 to your Squadron Hawk is a play I made.  2 to your Overgrown Battlement (+ 2-3 other damage) + 1 to your Lotus Cobra is also a play I made in testing.

It doesn't require any other targets to cast (you can always shoot yourself), it can't be mangled too badly by Spellskite (because of targetting restrictions), and it can finish someone off, which is often a problem for the deck.  It costs 2 easy to cast mana, which is also quite relevant.

Searing Blaze is probably the most powerful of the three spells.  It kills, or combines to kill a creature, and deal some useful damage to the head of a player.  It is an instant.  It combines well with fetchlands to be powered up on their turn.  It provides a key 3 damage, to combine with Bolt, or Volt Charge to take down the 6-toughness threats that exist (Titans, Sphynx).  It also pretty good at killing planeswalkers incidentally, and 3 damage is pretty good at taking down Jace, and several other planeswalkers, perhaps when combined with other sources.  Whatever Adrian Sullivan might say, incidental damage like SB can lower your threshold for victory, while accomplishing a useful purpose.  In that way, it really feels like Searing Blaze is a "two for one" when you can cast it.

The biggest problems with Searing Blaze are the mana cost (RR - the only double colored spell in the deck at present), and targetting restrictions.  You can't cast the spell, if the target player does not have a target creature, which can be quite problematic.  When you can cast SB, it's a 2:1.  When you can't, it's a 0:1.

Burst Lightning was in the way-back original version of the deck, and it performed adequately.  2 damage for 1 mana was nothing to write home about, and 4 damage for 5 was a fine end-of-your-turn play, but didn't do anything too crazy.  What I like about Burst now is that it is an instant, with multiple modes, so it can try to adapt to the killing them, or double-burning some threat, or removing an early problem creature quite well.

Being an instant in a deck full of instants is a pretty good deal.

Arc Trail often gets cast as a minor-Searing Blaze.  2 to your Mystic, 1 to you.  It is castable after a turn 1 Halimar Depths, or Island-Preordain, whereas Searing Blaze would have to wait until turn 3 after a turn 1 blue source.  Burst Lightning is a fine competitor for Arc Trail, in that it answers early threats, that have <=2 toughness (Fauna Shaman, Lotus Cobra, Stoneforge Mystic, or some aggro cretaure), while also going to the head later.  (and for cheaper)

Arc Trail is pretty amazing against creature decks.  Killing Cobra + Shaman, or Viscera Seer + Lacerator is pretty amazingly strong.   And when you get to the true swarm status, it becomes more of a necessity.  If Arc Trail got cut for Burst Lightning, there would have to be Pyroclasms in the sideboard for matchups like GW Quest, Kuldotha Red, Elves, and the like.  Currrently I'm sporting no Pyros main or side, because they're overkill with AT + Staggershock + 8 other burn spells.

Conclusion:

Since the matchup that this will effect the most is the swarm creature decks (Kuldotha Red, GW Quest), I'll try some games with 4x Burst, 2x Searing (2x Shatter) to round out the burn suite/empty slots, and see how those games go.  If I can pull 50+% with that configuration, it can only get better bringing in 4x pyroclasm.

Edit:

Arc Trail is pretty good.  However, the thing I have a problem with with it is that it is a sorcery.  Sorceries are really slow.  Sorceries don't kill in response to equip, or kill haste creatures, or kill flashed in 4 toughness creatures (in response to enchant even)

Burst Lightning combines with literally any other spell to kill an Exarch.  Burst lightning is very easy to cast, and acts like Lightning bolt, in that it gives you credible threats on 2, 3, 4 mana to kill Exarches, exponentially more than the omni-3-drop + sorcery arc trail.  Since the only answer this deck really has to Exarch is to kill it in response to equip, doing that as many ways as possible is quite important.