Don't listen to anyone who tells you one is better than the other for whatever reasons. Both tanks, when in capable hands, are equally viable and totally capable for any situation. Take it from someone who specializes in tanking, there is no draw back to either tank. Paladin may have block, but since you can only either block or parry, never both at once, block causes a diminishing number of parries. It's the "Oh, I can also block" mind set of paladins that is misleading, as the number of times you block also decreases the number of times you will parry an attack, meaning that for Warriors, every point of parry is worth more benefit than a Paladin, since blocking an attack is calculated before parrying.
The toggle skills for both classes are not quite equal, with Warriors receiving approximately 3% less overall mitigation from theirs, a number that is easily made up for with proper cooldown usage and wrath useage.
I will say this much. I never recommend for someone who wants to tank to start with Warrior. I recommend beginning with Paladin, since even a bad Paladin amounts to a little bit of something, where a bad Warrior is sooo much worse. I consider Paladin to be a class that tanks should try to graduate first before beginning Warrior, especially since Flash and Convalescence are two must have skills. Paladin will teach you the basics of tanking: proper CD usage, positioning, AOE dodging, MP and TP balance, all things that you really should try to master before becoming a Warrior, especially the MP and TP usage. If you can understand that, you can actually be a warrior that doesn't beg for Army's Paeon or Goads. Learn how cooldowns interact with one another also. This can make a tremendous difference in survivability. Bloodbath, on the surface may seem straight forward, but it isn't. Try combining it with Vengence in a large mob pull. Convalescence + Second Wind. The cooldowns on a Warrior may not seem as strong as those on a Paladin, but they have a shorter cooldown than most of the ones Paladins use, and have more synchronization, allowing you to get more bang for your buck.
Many Paladin cooldowns actually have the opposite effect compared to Warrior cooldowns, due to their Shield Oath. The damage mitigation from Shield Oath is calculated first, followed by the mitigation from your cooldowns. This means that the true amount of damage you mitigate with a skill like Rampart compared to the initial amount of flat damage is actually less than 20%. It's only really 20% of the damage that has already been reduced by 20%. While it's still mitigation, it's still a case of diminishing returns. Compare that to Warriors, where their toggle buff is actually able to boost convalescence's effect. Sure, warrior only gets a 20% healing bonus from convalescence as opposed to Paladins getting a 30% boost. However, that 20% boost is actually closer to 24%, since the healing bonus from Defiance is calculated first, increasing the base healing that is then boosted again by Convalescence.
So, contrary to the post above me, the edge Paladin has is actually blunted by very very poor cooldown synchronization. About the only cooldowns that Paladins have that combines well is Bloodbath and Fight or Flight, and even that is nothing when compared to Warriors combining Bloodbath with such skills as Vengence, Unchained, and Berserk. Especially since the damage increase from Fight or Flight is actually stunted by Shield Oath, where Unchained completely negated the damage reduction from Defiance, then when combined with Berserk, increases damage output the full 50%. Coincidentally enough too, they both last 20 seconds, and Unchained has the same CD as Vengence, and Berserk has the same CD as Bloodbath. My normal cooldown usage for big trash pulls switches between Foresight, Convalescence+Second Wind, Awareness, on one, then Bloodbath+Vengence, 10 seconds into Bloodbath, Infuriate+Unchained (If I don't have full Wrath to begin with) and Berserk. By the time two large pulls are done, my first set of CDs are ready for use for a third pull, and I just switch back and forth.
Also, I am not a hypocrite. I followed my own advice and raised Paladin first. In fact, Paladin is my main class, and I have used it the most, and I know it the most. It is my most cherished class, and I only advanced into Warrior after mastering Paladin. I am still mastering Warrior myself, and still discovering its capabilities. I only recently realized how strong Bloodbath+Vengence is myself. I don't boast about Warrior's synchronization out of some biased love for Warriors, since I truly love Paladin more. I just simply recognize how strong Warrior really is in capable hands. Do not for a second consider one less than the other for any reason. The real difference between them is the player. Anyone who says one is better than the other simply doesn't know what they are talking about.
TLDR: Why not both?