Corgi Doubts

The main reason Corgi isn't that populair is because he is aps-based. This is actually fine for TT, but very bad for most instances which now deals with anti-aps buffs, such as FSJ, UP, DH, etc. The reason why harpy is so populair for PvE is for that exact reason - shes not aps based. The Harpy along with the Battlebot, are the only two venomancer pets that have spammable skills thus do not rely on aps'ing mobs, and are therefore considered actual damage dealers in all instances.
I think Corgi is a great fit for TT - but that's about it. Spending that amount of money on the pet would take you forever to farm back purely done in TT - so why not spend the money on a pet thats useful in even more instances?
Phoenix was my favorite choice for farming TT (basically just a weaker corgi in all aspects) but I just dont think Corgi, nor the phoenix, is worth the investment anymore.

I mentioned Harpy and Battlebot as damage dealers. These two both comes with pros/cons over eachother too of course.
Battlebot is a lot tankier than the harpy, and he hits slightly harder (but slower). He deals physical damage.
Harpy is squiser, have slightly higher dps, but comes also with spamable AoE unlike the battlebot. She deals water and wood damage, and its also possible to do physical damage.
They both come with PvE-only AoE stuns as well, where the difference is battlebot stuns everybody around himself, while Harpy stuns everybody around her target.

Personally I think Herc is a great choice too if you need to rely on the pets tanking abilities. I still use him from time to time, but Harpy is my #1 choice these days.