I disagree with Jonah on this. It might make sense to avoid topics which few educated people find interesting, but beyond that, it makes no sense to choose fields of study by their research popularity (as opposed, say, to their appeal to the potential student, their relevance to possible careers, or the availability of good materials).
Even if it did make sense to do that, dismissing mathematical logic because relatively few mathematicians work in that field would be silly. Much research in mathematical logic is now done in computer science departments (albeit often by people whose PhDs are in maths), because of the importance of logic as foundation of computer science. Many mathematicians are relatively ignorant of logic, because for historical reasons (and because there are few mathematics professors pushing for courses in it, because they have skipped to CS departments!) it is often neglected in undergraduate mathematics syllabuses. Neither of these facts makes mathematical logic any less suitable for study by a mathematical youth, however.
