Principal Research Scientist vs. Senior Research Scientist?
According to the MIT EECS pages, there are two tracks:


Academic Research Staff.  These are mostly shorter term (such as a Postdoctoral Fellow), and are funded externally (by grants and similar).
Sponsored Research Staff.  These are longer term appointments which are funded by the department.


Senior Research Scientist is an Academic Research Staff position, because while it is long-term, it is also a position for which the researcher is expected to procure their own funding.  

From 5.3, the SRS's funding:


  Appointees to this rank generally are expected to raise funds to
  conduct research and cover their own salaries.


Principal Research Scientist is a Sponsored Research Staff position, and is funded by the department (though I'm sure they can get their own funding also, their salary is paid by the department).

From 5.2, the PRS's funding:


  Funding responsibility resides with the department, laboratory, or center that is the locus of the appointment. 


From 5.1 on the difference:


  Two types of research appointments are described in the following
  sections: long-term appointments, which are classified as sponsored
  research staff appointments, with the exception of the Senior Research
  Scientist/Associate/ Engineer position; and short-term appointments,
  which are, in general, classified as academic research appointments.
  
  The purpose of the campus research appointment structure is to create
  professional sponsored research staff categories with employee
  benefits and privileges in order to encourage long-term research
  careers at MIT. The concept of long-term professional careers,
  independent of classroom teaching and supported entirely from research
  grants, is not a new one for the American research university. This
  mode of research, with professionals working in departments,
  laboratories, and centers, is an important mechanism for universities
  to maintain research viability, enter new fields, and bring
  researchers to the campus.


It seems evident that Senior RS is basically the same position as PRS, but not funded by the department.  In some cases, the PRS could earn more, because (for example) a NIH-funded SRS would be limited by NIH regulations, while a PRS who is funded by the department is only limited by whatever MIT chooses to pay them.