Well, pets earn experience using a parabolic curve, so pets at the peak of the curve earn the most EXP while other pets on either side of the peak earn less. Versus Pandaria, Northrend, and Cata tamers (and soon to be WoD tamers too), pets that are levels 10/11 earn the most EXP per battle. Given that, you want to use the 50% food buff on pets at 10/11 or closest to them as you have available. The method I found fastest (assuming you are using one toon):
1. Wild pet battles with safari hat for pets until level 10 (since not daily, this can be done as often as needed).
2. On days I have the both the 25% an the 50% food buffs, use carry pets closest to level 10/11
3. On days I only have the 25% food buff, use carry pets above those in group #2 but only up to level 23
4. Wild pet battles for my level 24 pets to finish them off.
Long term this makes the most gain out of the food buffs. It's a weird feeling system though. Some days you get no pets to 25, other days you get 15 pets to 25. But for long term, it is fast.
Mind you, I am doing 17-19 tamers per food buff hour (11 in Pandaria, 4/5 Northrend, 2-4 Cata) depending on RNG and whether my son tries to hand on my arm as I play.
Carry Pet Experience Reference Guide: