An average Spriggan has at least the capacity of a city-level scale, from Invel's blizzard to Ajeel's sandstorm. Neinhart, before his enchantment was able to cast his Historia across Hargeon town. And after getting enhanced with enchantment, Neinhart was completely immune to Brandish's magic, however he was defeated by Natsu in a single hit. There is a limit to who Brandish can apply her magic to. Her magic was used to heal August by shrinking his wound from fatal to negligible. August allowed that to happen because he knows Brandish wanted to save him. It's unlikely her magic stands a chance to have an affect on August if he actively resists it.
If we are going by feats, rather than hype or portrayal, Laxus definitely has greater magic power than Brandish. Laxus during the fighting festival arc, already has reserves of magic power that goes beyond city-level. He surrounded the entire Magnolia town with Thunder Palace and organic link magic, 4 of which orbs were enough to eventually put Erza down and out. He fought Mystogan, Erza, then Natsu and Gajeel in quick succession with little to no rest in between any of those fights, casted another city-level spell in Fairy Law, and towards the end was still outclassing Natsu who got a second wind from his flames of emotions.
Brandish's magic, like Larcade's curses and Dimaria's age seal, are no doubt very tricky to face because these spells/techniques tend to ignore conventional durability. Due to this, the potential to extrapolate their abilities is always there. Especially when you have other characters like Dimaria making offhanded remarks that Brandish can simply burst her any of her foe's organs. Despite her never actually doing so in the story. Furthermore, Brandish's character has never fought in such a manner.
Laxus was hyped by August of having the power which could threaten to defeat and kill a Spriggan when he nonchalantly dropped a casual nuke without batting an eye on Ajeel. Laxus' infection from the deadly concentration of magic barrier particles was so bad he suffers from recurring heart attacks and violent body spasms, in conjunction with Wahl's immunity to lightning magic, greatly hampered his fight with the latter. Once those two massive handicaps were overcome, Laxus killed Wahl by tearing through him. He actually delivered on his hype, and is the only person to defeat a Spriggan with such power that they were killed (for reference, the only other person to accomplish that is Acnologia).
Tldr; voted Brandish. Because titties.
