While you've mentioned two ways that may seem a bit extreme, I'd say that most of the acknowledged players keep levelling pets the classic way up to 19-20 lvl, keeping their stones for the several last levels (note that these require more experience and are also prone to "overtraining").
Myself, doing that (and adjusting the experience gathering to get the original level on its 0-20%!), I have 600+ universal training stones and two unused ultimate (purple) ones.
You can also keep in mind that if you level your pets on a non-max level character, that toon gets experience as long as your best pet is up to 5 levels above the battled ones.
The easiest way is just battling the 25s with a levelling pet and it also gives the best results for a player. Not very encouraging to visit the old areas!