Choosing a PhD field, based on short term or long term decision?

I saw already questions regarding field choice, but they do not explicitly answer this question :

Choosing a PhD by following short term interest or following the long term vague idea of what one would like to do or be in the future ?

To explain what I mean, usually choosing a PhD field is restricting, it is harder to keep interdisciplinarity or swap of field if they are not seen as "close". One can be highly motivated by a subject A now but do not see a clear future, contrary to a second subject B a bit less motivating but where future is more predictable.

To take example in my case, I am considering two subfields of Computational Engineering : Physics-based animation in Computer Graphics and FSI in Computational Mechanics as PhD fields. Since one usually considers these two fields quite appart, I do not think I will be able to switch area after the PhD. I am motivated by the former but it is hard to catch a glimpse of future possibilities contrary to the latter because engineering seems more of a "safe" choice considering employment.