All haze bomb lines have terrible range until you get it to 5 stars, at which point their radius practically doubles.

Freezing vaporizer is the superior bomb in almost all circumstances. The fire bomb line is the highest damage, but the freeze line offers unmatched control. Unless you're planning on being a bomber, there's virtually no reason to get a fire bomb.

I would absolutely get both (actually, I have both). Both have, in my mind, equal utility. Freeze is great for sticking enemies in place. This means gun puppies and retrodes can't aim at your party, lumbers can't turn and smash you, etc. However, when the enemies do a circumferential attack, especially one with a long range like lichen spikes, ice isn't nearly as useful because the enemy can still hit you and there is no way to "sneak up behind." Fire is fantastic against enemies with range attacks because it does huge damage. It's also great in arenas (not flamelash, obviously) and other crowded rooms because it does massive damage to lots of creatures - enough to neutralize the healing power of ironclaw menders. I never go in a party without both of these bombs. (I usually carry freeze and my mate carries fire, so you don't need to take both personally if you have a friend who you party with regularly. Despite that, Ash is actually my favorite of the two. I just worked up freeze first, so it's what I use most.)

As stated above, haze line bombs are most powerful at the 5* stage, but contrary to Metaphysic's warning, they can actually be quite useful at the 2-4* levels as well. With the smaller radii strategy is more tricky, but in my opinion maybe a bit more fun. Look to use them in hallways or other narrow openings that force enemies to walk into the mist. If you're good at "kiting" (i.e. getting enemies to follow you), you can lure things into the mist no matter how small it is.

Shivermist Buster:
Provides Freeze. Freeze prevents monsters from moving - this includes turning.

Pros: shuts down enemy groups, allowing you to single out priority targets; controls each new wave of an Arena/Danger Room; puts out Oilers; keeps Gun Puppies from tracking players; frozen Phantoms cannot phase out of Freeze; is one of the best loadouts for Vanaduke.

Cons: frozen enemies become obstacles; Freeze does not prevent AoE attacks.

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Ash of Agni:
Provides Fire. Fire is the single strongest DoT in the game.

Pros: deals strong DoT to targets that are too strong to deal with in close combat; can mitigate monster healing.

Cons: outside of dealing damage, is not useful; sets Oilers on fire.

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Conclusion:
If you like taking control of a situation, take Shivermist. If you like dealing damage, take Ash.

With the recent upgrade to jellies, I'm rapidly reaching the conclusion that in the absence of Oilers, the Ash of Agni is the superior choice in most situations over the Shivermist Buster. Not that the SMB isn't an awesome weapon that can totally trivialise some pretty tough enemies, but generally kiting with the Ash results in stuff dying much, much faster.

There are times, however, when the SMB is a godsend. It's a bit situational, and might depend on your party makeup to some extent, but it'll be useful enough often enough to be worth having both. It might also be more useful to newer players, allowing them to concentrate more on offense without fear of retaliation or getting swarmed.

If you have a friend, or a group of people you regularly run with, it might be worth asking them which one they'd prefer you to be sticking to the floor. The Haze-lines are a support tool, so they might prefer one type of support over another.