The first thing is to always use your highest level available pet, regardless of the type. If you have multiple pets to choose from then it's a question of what type of XPing you're doing. Fire is very good for pet chaining against low blue cons while solo. Earth is good for buffing to the gills with a Cleric partner (especially when CH is a factor) and just keeping it hasted and DSed while it tanks everything. Water is good for max DPS while grouping. Air is good for grouping in dungeons that have a lot of casters.
In general, fire pet will always be your riskiest pet, having the lowest HP in exchange fort the huge DS. Earth is your safest pet, with the most HP and the root proc. So if you're solo or duo choose between them depending on how comfortable you are with the mobs you're fighting. If you've got a lot of experience at a camp and a good understanding of the mob levels/hp/damage/etc... and just want to burn through them as fast as possible then lean on the fire pet. If you're trying out a new zone or aren't 100% dialed in for whatever reason, take advantage of the extra beefiness of the Earth pet for an added margin of safety. And then most of the time in a group just pull out the Water pet for maximum DPS and a little offtanking as needed.
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