Ash of Agni vs. Shivermist Buster
Which one, and why?
DPS or CC, personal preference.
Get both, you won't be sorry. But get a Shivermist first, useful in more places. Also, are you a swordie or bomber? What is your current/planned equipment?
Further reading with lots of in-depth analysis:
http://forums.spiralknights.com/en/node/6207
http://forums.spiralknights.com/en/node/2964
http://forums.spiralknights.com/en/node/11495
http://forums.spiralknights.com/en/node/12880
Currently I'm a full vog with grey owlite, sporting a DA, Levi, Peacemaker, and hopefully one bomb. I'm trying to decide between the fire and ice effects -- mostly swordie though, and I feel like a haze will be better than a nitro. Any other suggestions?
The preference depends on your style of gameplay and experience. If you feel overwhelmed by the amount of mobs on the screen, get shivermist, which is a most excellect bomb. If you like being overwhelmed by mobs and just want to kill them faster, go with ash, which is a most excellent bomb.
PS: Shivermist will be of use in FSC whereas Ash will not. I believe shivermist may be a better starting bomb.

Definitely Shivermist first. Unless you're in an ice-themed level (where most of the monsters are immune to freeze), it will save your hide plenty of times. I would say get the Ash only if you decide to become a bomber knight or have already maxed out your other weaponry.

I crafted my shivermist buster yesterday and it is SOOOO worth it. Also, read my topic about: Pure Amazing. I couldn't believe this
To find out more. AoA is much worse.
Depends on your play-style. I would go with Shivermist because of its support capabilities as Ash of Agni won't get you out of any tight spots.