Before I had the money to buy my own guitar, I borrowed from friends. One of them had a 12-string that she let me play. I love the sound, and I will always have at least one in my collection. That said, you should probably get a 12-string after you've had a 6-string for a while. A 12 has a wonderful sound to it, and it isn't that much harder to play, but it is just different enough that switching back and forth can hold you back a bit in your progress. Another thing is that while a 12-string adds good spice to music, it soon loses it's appeal if it's all you have. A lot of songs just don't work very well on a 12, and that limitation can be frustrating.

I suggest getting a good 6-string first, and then adding a 12-string later. I think you will find that you really enjoy having a 12 available for special songs, but if you had to get by with just one guitar, it would be a 6-string.