What the heck... I am going to add my two cents as well, mind you that is in US currency as even Canada is outpacing us now.
As someone who just returned to WoW after almost 2 years away, and now with the Horde since I talked several RL friends into rerolling and getting away from the Alliance kids (could not resist...) I just recently rolled several new classes and to date I have a Druid at 27, a warlock at 26, and a mage at 17. In my prior career I was a priest, paladin, hunter, and rogue so wanted something a bit different to go with what I already new... all of the above were either 70 or 68, by the time I quit.
So... more to the point, I wanted something that would top the dps meters and be fun to play and going off what I knew 2 years ago is how I determined that I wanted to try one of the above 3. I have stopped playing the warlock for now entirely. Between the mage and the druid at low levels, the druid is by far the better choice IMO. Easy to get groups as healer or tank or off dps for that matter... survivability is incredible, and downtime is almost non existent.
While I realize that a mage can do sheer dps and once evocation is available, their downtime is reduced significantly, from a standpoint of Moonkin or mage... the druid is going to win out 9 out of 10 times.
Before I get attacked in this thread as already forewarned, I also realize that this discussion is for at least lvl 40 and above. Going back on my previous history, and there are significant changes obviously, I think it comes down to what you need in your 10 man or 25 man raids, and quite honestly, the same for 5 mans. A balance raid or PUG is required to be successful. In terms of one is better than the other, both have utility in all situations, but a Druid has more utility. In regards to PVP, it is way more about gear and personal ability. My shadow priest used to own just about every class in world PvP, but in the arena... 2v2... yea, I died... again, though, look at the situations. As already mentioned how often is someone in an "ideal" situation for the whole time. In world PvP, I was normally the one making the attack first, as that is what makes / allows for my survival. As a cloth wearer squishy, my first attack for 2k was the difference 90% of the time.
Regardless, why... the only answer is personal and guild preference. Everyone has an idea as to what is the best, but often you may be shaving 20 minutes or less of a successful run, and honestly, the run is successful either way. Yes, I realize the elitists out there want to have the fastest runs, but that is also the joy of WoW, some guilds play to have fun and win, some just want to win, and some just want to goof off, find one that works for you. Enjoy the game.