My best recommendation is that you take a look at this article: JR Pass (Japan Rail Pass) - Everything you need to know
There they explain who should buy the JR pass, who shouldn't and there's a video detailing everything too.
But in my own personal opinion, it seems like you'll be using the Shinkansen only 2 times, right? If so, then you don't need the JR pass. But if you use it maybe 3 or 4 times, then it is definitely worth it!
I hope this helps!