While I agree with most of Fulltilt's advice, my take on pets is very different.
Fulltilt wrote:
At lower levels, Fire pet is probably your best choice, followed by earth (root is your friend when soloing). Fire pet's DS saves you from haveing to cast another spell yourself
The fire pet
does have the greatest damage output, pre 50. In sub 30 levels, however, I generally found it had too few hitpoints so only used it for group situations. From 30 to 50 I used the fire pet almost exclusively, but had to get used to casting a lot more heals than with the other pets.
The earth pet, in my opinion, is the worst choice for most situations. This pet has the lowest agility of all the pets, so it rarely avoids a mob's swing. This is why he has the most hit points. The earth pet also has the lowest damage output of all the pets. Instead of casting a damage spell like our other pets, he spends much of his time getting hit while he waves his arms in the air, trying to cast his root.
I believe the fact that the earth pet casts root retards a young magician's aggro learning curve. Do NOT rely on root spell to keep a mob off you.
Learn how to nuke effectively and not pull aggro off your pet/tank. This is one of the most basic and important things a magician must learn.
Your pet is not only your creation, but also your training buddy.
Many threads list the water pet as the best choice for lower levels, and I have to agree. It has good HP, very fast regen, and is very poison resistant.
I know I'm in the minority in that I used my earth pets less than any other magician around, but I do believe my reasoning is solid.