Hi Noble, it really depends on your career goals. If you are planning to stay in academia, selecting a different area within your dectoral research field would be better.
This is usually mutual. While graduate students are looking for good labs for post-doc positions, the PIs (principle investigator, who put out a post-doc position ad), on the other hand, also want to select a good candidate. And, I believe that most PIs want to choose a candidate who at least has some 'experience or knowledge' in the funds-supported projects. This can facilitate the progress of the projects and finish the projects in time. A quick finish of the projects means that they can present the data, write more grant proposals and hire more post-docs. So, when looking for a post-doc job, if your CV demonstrates that your expertise is in a specific area and you submit your CV to a lab of 'entire different field', most likely you won't get a response from the PI. I am not saying that you definitely can not get one, but the chance probably is very low. Remember, the competition for a post-doc position in a good lab is pretty tough these days too.