which class is more needed ? None. They are both good and every good group (aka group that can handle mechanics) have both or can have both (some prefer double pld for some obscure reasons)

Which class is preferable ? The one you'll enjoy to play, obviously. Which also answers your 4th question

Which class is less played ? Was WAR in 2.0. Way less accurate to say that since 2.1

Side note : don't say you're a "really good" paladin or warrior when you are lv 30 on one and 5 on the other ^^ Wait for the difficult content to come, as this is no WoW.

Also, WARs may have more damage, but they don't have more aggro than plds. On solo target they are equal. Also, saying that WARs have more hp (thus making better tanks because more survivability) is only true until pld lv 40, as you'll get a perma-20% damage reduction which totally nullifies the advantage of more hp. Also, this difference makes warriors more difficult to heal as they need more heal than paladin to get on top.

On the same side, your statement about healers have to be reverted WHM are PLD's best friends, and scholars love warriors (lustrate heals for a good ammount on warriors). But in the end, it's mostly a matter of habits.

Other (slight) thing : pld's heals are totally worthless, better take stoneskin (lv34 WHM) and forget about the heal.