If you don't mind the extra time to tame a hard to find/get wild pet, then I would definitely use it for one that cannot be gotten as a rare. Then it all goes back to what pet you like best; and whether 5.2 might be worth waiting. I don't think 5.2 is that much of a wait. Have we gotten a date on it...

If you don't want to sell it and use it on one of your pets, I suggest using it on one of the harder pets to find. Three come to mind right away: Scourged Whelpling, Baby Ape, and Minifernal. Provided you plan to get those pets.