Ret has been acknowledged even by Blizz as the "leveling build" for paladins, which caused end game ret to get angry. You can kill stuff faster, which makes for more xp, which means you get kill/collect quests done faster which means more xp . What does faster xp mean? You guessed it! Faster leveling /gasp!!!!
The Prot tree offers nothing that helps kill faster until you get towards Reckoning and Holy Shield. Survivability is great but in no way reflects towards faster leveling. However 35+ after reckoning it does pick up and levels quite effectively and is competitive with Ret, though ret will still have a slight edge. It also allows you to tank, so since it is in a virtual dead heat with ret 35+ lots of pallies go this way so they can tank dungeons. DPS is entirely dependant on how many mobs are hitting you, so there are situations where you will level slower, some where you will go faster depending on zone/mobs/quests but it evens out.
Holy has absolutely no gear that compliments it until lvl 58+ which sets it at a SEVERE disadvantage for 48+ levels. It also has very little in the way of upping damage done, so you kill slower and earn xp at a fraction of the pace. So while doable it is not the most desirable build for leveling.
As for Divine Str in the Holy tree it will and always has been a very poor choice compared to the alternative of Divine Intellect. Since the majority of your damage either as Ret or Prot will be Yellow damage coming from your mana pool the increase to Int will always outweigh minor gains to AP. Also boosts survivability and ability to grind for longer durations with less downtime thanks to an increased mana pool.
____________________________
Bode - 100 Holy Paladin - Lightbringer