I would start with the lower of the two paces then see how the first couple MP runs go. If it feels too easy and you recover quickly then try the faster of the two and see how it impacts you. You should feel worked out when you're done, but not gassed and a grinding finish is a sign it's too fast.
I'm probably not the best person to answer the MP question since those were my biggest issue, but since I just went through popping my cherry I should probably write what I did and what I think I should do differently next time.
Ideally, I'd like my cool down to be one mile. So if I get my mind right it'd be 6 miles easy, 8 at MP, then a one mile cool down. In practice, I found it exceptionally difficult to get locked in to the more intense pace of a MP if I ran easy for 50 mins first. I tried to anyway because I was too fixated with the short cool down and as you may imagine it didn't go well. I'm probably not doing pfitz next time, so I don't know if part MP/part easy runs will be part of the plan, but if they are then I am doing the MP as soon as I'm mentally ready. In the end there are two important components with a good MP - #1 getting used to your marathon pace and #2 time on your feet. It makes no difference if you do the MP miles 2-9 or 7-14. So long as you do them and do them well. That said, you must get used to running 2+ hours, so the time on your feet is just as important. Your body needs to practice using fat as fuel and the only way you can achieve that is by running for more than 90-100 mins'ish. It may not be the most mentally engaging finish to a run, but if you do the MP's 2-9 do not cut the run short - do all 15. Otherwise you will only get half the benefits of the training run.