Visit Takayama or Nikko?

I'm considering a day trip from Tokyo with the JR pass. Would you suggest i visit Takayama or Nikko??? Which will be more interesting? Prior to that trip I will be visiting osaka, kyoto & hakone area anyway.

. . . not a day trip from Tokyo. The usual way to go there is Chuo Line to Matsumoto (better reserve a seat) and take a bus to Takayama. That will take you maybe 5 hours.

but try to leave for it as early as possible so you have a full day to climb up to the temple and explore! (or stay overnight there at a local hotel)

We have only been to Takyama. It takes about 5 hours one way using the JR pass - you need to stop over for an hour in Nagoya (which you tend to use up looking for which platform, buying stuff from the platform vendors, etc)

Takyama is awesome for the mountain area about 1.5hour one way bus. A number of buses a day but not continous and not part of the JR system. Here, you can enjoy outside onsen amongst the changing foliage. The town of Takyama is picturesque however after spending time in Kyoto, Himeji, Kanazawa, etc we were ready for a change and didn't spend a lot of time exploring the museums and shrines. Its a beautiful area but takes atleast 9-10 hours from Tokyo station to Takyama station (not inlcuding getting into Tokyo station).