Probably best plat purchase whatever has the best return of gold. I believe ankhs are the best gold return at the moment.
Then buy pets that are good AND a good value. Haven't really played in a while, but I believe Grimm and Dova are on the cheap side--especially considering how good they are. And don't overlook some cheap & basic but powerful pets, such as Ethyl.
Don't fork over all your gold & platinum for an arcane pet. It'll be better used to acquire gear that'll boost your stats higher & for considerably less gold.

Tip: play on the cheap side for a bit, while you amass your financial moves. The game is more fun. You don't need to be running around with mythic pets and elite legendary gear while clearing out Kraken maps at lvl 22. Using overpowered gear early on has a major drawback, too: you never learn how to master your character and how to play it.