I think that people are kind of arguing two points here.

Warriors are the better tanks at this point of WoW. However, the issue is, how much "tank" do you actually need for leveling purposes? In which case the ease of use and simplicity and effectiveness of the Paladin comes into play. Paladin is good enough for all 5 man classic content, and in a lot of ways superior due to not needing that superior raw "tank" for the content we're talking about. They have better aoe threat generation for one.

We can present a few debateable edge case scenarios about what a warrior can and a paladin can't do, but the greatest portion of your time leveling up, the paladin being fire and forget threat magnet, is going to make the process less of a headache for most people. If your goal is to have an actual end game tank at 60 you're going to raid with, then level a warrior. If you just want the easiest path to 60, I'd suggest using a paladin to tank.