There are many many MANY topics on this same subject.
On this note, i would like to remind everyone that, before opening a new topic, a good old "search" doesn't do harm
. More often than not you will find that your subject is already under discussion and you can simply add to it, instead of opening a new discussion altogether.
That said, no mage spec is particularly shining at the moment. Moreover, the general consensus seems to be that in order to perform adequately in comparison to other classes all 3 specs depend heavily on their BiS legendaries.
Which, again, leads to the best and simplest advice i can give: try out all the specs, choose the one you like the most and STICK TO IT NO MATTER WHAT. You will be rewarded in time.