If you're going to be there for 2 weeks and considering going around on the trains I'd suggest looking on buying a rail pass
You can use it to get to Tokyo from Narita, and any JR rail is free from then on. It'll save you money and gie you the chance to go wherever you please, whenever you please! I really enjoyed mine... I got one for all of Japan for 3 weeks, and therefore got to go up to Sapporo and down to Hiroshima! I liked the trains... but man, terrible coffee