I'm a first time applicant, so this is just how I interpreted the solicitation. For the research proposal, I mostly wrote about the implications of the project & how the research could benefit science as a whole (ie the intellectual merit of the project). I also briefly mentioned how my specific research background fits into the feasibility of the project. For the personal statement, it was much more a testament to my own research background and skills - I wrote about each lab I worked in, what I got out of it, how it helped me grow etc. and ended by talking about how it fits into my career plans & what I can contribute to science. Again, that's just how I interpreted it! I'm sure there are other successful ways to approach this.