I have Voltedge, Combuster and Acheron. I also have Divine avenger and Faust. (yeah I mean 4*) I did research on Voltedge and Divine avenger to see which was better. They both had ups and downs. I know most of you would say voltedge is better but DA can be very useful in tight situations.
Which is better?
Votes: Brandish 1; DA/GF 1
Each is situational...
DA is good as a soloing weapon and as a group Guardian weapon. It's attacks and charge knockback enemy hordes and deal great damage. Keeping back the monsters is your main priority with this weapon, be patient and think your actions out thoroughly since you can affect large numbers of monsters. (It is the weapon of choice when fighting the Roarmulus since all the charge attack projectiles hit the boss.)
GF is only really good as a group guardian weapon, and even then it isn't too good considering it's charge attack is terribly risky. It is best left as a PVP weapon. Still... it is one of the few shadow damage swords out there.
The Glacius is an interesting brandish that veers off from the typical brandish tactics. It's charge is incredible defensive and when in a smart party can be used to set up devastating moves safely. (unless you travel alone or your party is patient don't rely on the status to deal damage.
The Combuster is a good reliable brandish that packs a good offensive punch and when used correctly can keep enemies back (though no as well as the DA).
The Acheron is similar to the Combuster but stick to shadow damage, having one means you have a reliable shadow type weapon to use.
The Voltedge is more or less a cross between the Combuster and the Glacius. It deals less raw damage than the Combuster (but more than Glacuis) and its status has less holding power than the Glacius.
There is one brandish that you didn't mention and that is the Obsidian edge. Not as powerful as it's brother the Acheron, however it's status is extremely useful. Halving damage you take and increasing damage given as well as reversing any healing means that this sword has far more utility as a support weapon.
(not even going to talk about the Cautery sword until it gets a 5star version)
I can't really say more than what's in my guide that Krakob graciously linked. Brandishes are usually better overall, but DA/GF are probably more fun.
I haven't found anything more fun than shoving an +800 hit down a Slag's throat with a DA charge then watching him get flicked around like a ragdoll.
(nitronome tennis comes close)
XD
This question has been asked plenty of times before. To find answers from previous threads, you can use google to search something like elemental swords site:forums.spiralknights.com on Google.
I can also recommend Bopp's sword guide, which covers roughly your question in the elemental swords section.
I would recommend Combuster over Voltedge. Since it has fire, the knockback is predictable and a bit safer, while it will also output more damage since the enemies get hit by all explosions in the charge, provided you aim correctly. This is not the case with Glacius and Voltedge, which may keep your enemy closer due to the statuses.
As for picking a Brandish or Divine Avenger, I recommend Brandishes. Their charges are incredibly powerful, while the damage per second of the combo is very high as well. Divien Avenger can be useful for when there are lots of enemies as you could be hitting lots and lots of them, as opposed to Brandishes which struggle with three enemies. If you're using Auto Target, you may struggle even with hitting two enemies at once.
The Divine Avenger charge is quite sluggish, although it is powerful. It's situationally useful but it's not something you'll want to start in the middle of a battle due to how vulnerable you can become. Another problem is that the projectiles will not have any effect if you use the charge while next to a wall, which is not the case for Brandishes.