Depending on who you order from you get different 'rewards', for TSW this seems to take the form of pets (much like leets from AO) except that these pets actually offer a low level buff.
They are:
Cat - HoT
Wolf - Damage Boost
Hound - Dodge Boost
Since there arent any details of the buffs themselves its kind of hard to decide between aesthetics and usefulness

Myself I'm quiet partial to the wolf, however for any kind if low level dueling is even remotely similar to AO the Dodge / HoT are probably going to have far more effect?
Also on a side note from what i've seen on the forums you can preorder elsewhere (i.e. amazon.co.uk) to get the pet you want, apply the code to your account then buy things such as the Lifetime pack through the funcom account screen.
ps. Does anyone know if there are any pet "classes"? (ok I realise i should say "builds" but old habits die hard!)