I assume you mean "Phonetics and Phonology" referring to that particular sub-field, and not "Phonetics or Phonology, or both". P&P refers to an approach to studying sound systems that treats sounds symbolically (so you don't focus so much on duration, formant frequencies and so on), but which attempts to discover the functional underpinnings of many traditional phonological generalizations. A classical example is the question of why voiceless consonants are more common than voiceless consonants, and a study of the physiology, acoustics and perception of voicing provides an explanation.
There are a huge number of areas that are treated by P&P, and market forces change rapidly to the point that it is unwise to set out to study a specific problem, when in the 6+ post-BA years required to get the terminal degree, things could change and it might prove unwise to invest in a dissertation on duration patterning as a correlate of metrical prominence.
At the undergraduate level, though, it is really unwise to focus on P&P; instead it would be best to learn the fundamentals of phonetics, or phonology, and experimental methodology, and maybe do some small demonstration project such as reporting whether vowel duration in Norwegian monosyllables is significantly influenced by whether the words ends in a vowel, in C, or in CC. (I suggest Norwegian since that experiment may not have been done, whereas for English it has been done to death).