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**Better is a Matter of Taste**

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I believe the answer to this will lie in  what it is you want your creatures to do with the flame produced. The main difference between liquid and gas is, of course, *volume containment*. Gasses spread, diffuse, very easily once exhaled into the atmosphere. Liquids can remain somewhat cohesive for a quite a distance.

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Consider how a person's breath looks when it's exhaled on a chilly day. All that white frosty "steam" is frozen water vapour. Once it leaves a creature's mouth, it spreads widely and dissipates quickly. There's no way to control it beyond a couple feet. This is perfect if you'd like to use gaseous fire for display purposes (hey! I'm really hot! Come mate with me!) or for communications (so, that sort of blue tinge to the flame indicates that speaker is using the subjunctive mood, rather than the previous orange of the indicative). It may not work so well over a distance.

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Now consider how comparatively easy it is to control a stream of fluid. Liquids are more cohesive and can be expelled over greater distances. Even a small object like a water pistol can shoot a stream over several feet. Something more powerful like a flame thrower can multiply that to tens or even hundreds of feet. This may not work so well when all you want to do is keep your hatchlings warm on a chilly night!

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So as for feasibility: both are feasible.<br>
As for suitability: this will depend on the application. There are good arguments for either or both.

As for your knock-on considerations, I think energy density of various flammables ought to be discoverable by googling. Considerations of available volume, bladder capacity, recharge time, etc.: those would make for good follow-up questions, as those are matters of anatomy, physiology, and biology within the context of fantasy creature design.