There is a recent analysis in Language Log (here) of linguistics dissertations submitted and job opportunities advertised, based on work done by Stephanie Shih and Rebecca Starr. The results look like this, where the blue columns show the number of dissertations in each field and the orange and yellow columns show job opportunities.
Basically, for someone starting their academic career, you want the blue column to be low (you don't want too many competitors) and the other two columns to be high (lots of jobs).
Guess what: for mainstream linguistics, you are best off in phonetics (the leftmost category). Morphology, syntax and semantics are not so good, and sociolinguistics is also not good. Psycholinguistics looks OK, though.
