JRPass

14 day or 21 day?

Hi,
Im travelling to Japan on my own at the start of November. Doing lots of sightseeing and visiting friends.
My trip is only 19 days long but doing a lot of travelling. Should i get the 14 day pass from the 3rd day to the 15th then just pay for the 2 trains i need? Or buy the 21 day and not worry, or even not get one at all? Also as im travelling will have a large rucksack will it get in the way on trains?

My plan is as follows:

• Day1-3: Arrive at Narita and head into central Tokyo and stay there.
• Day 3-5: Stay in Yokohama and explore area, heading back for a concert in Nakano district.
• Day 5-11: Head down to Osaka from Yokohama and stay in Osaka, day trips to surrounding area of Nara, Hiroshima,Himeji and Kobe.
• Day 11-15: Head back from Osaka to Tokyo and stay there.
• Day 15-18: staying with friends in Yamanashi
• Day 18-19 Back to Narita.

Please help me many thanks

Eve