14 or 21 day pass?

Hey!

I'll be in Japan for 21 days, and am planning to spend the first few nights in Tokyo, with an overnight trip to Mt Fuji and a day trip to Nikko, before heading to Osaka then Kyoto then Hiroshima and then back to Tokyo. Does it make more sense to get a 14 day pass (and activate it when leaving Tokyo for Osaka) or get a 21 day pass (and have the cost of our train from Narita airport to Tokyo, plus the the day trips already covered)?

Thanks in advance!