I'm a resident of Hokkaido.

Which path you advice us for night train

There are two available limited-express night trains. They are called Hokutosei and Cassiopeia.

Both of these night trains run along the same path:
Tokyo - Omiya - Utsunomiya - Fukushima - Sendai - Morioka - Aomori - Hakodate - Muroran - Tomakomai - Sapporo

http://jprail.com/trains/overnight-express-train-cassiopeia.html
http://www.jreast.co.jp/e/routemaps/cassiopeia_hokutosei.html

When moving from Tokyo to Sapporo, you have no other path option except following along this path.

Due to the fact that you can't easily reserve places on Hokutsei or Cassioepeia in advance with a JR Pass from outside Japan I think they will highly likely be booked out before you arrive. Also note that you have to pay a surcharge to use either of those trains with a JR Pass anyway.

You have two other main options with a JR Pass.

1 - Take Shinkansen from Tokyo to Shin-Aomori and then catch the Hamanasu night train from Shin-Aomori to Sapporo. I think the Hamanasu is also likely to be booked out but it may depend a little on your dates.
Good explanation of all night trains:-
http://www.japan-guide.com/e/e2356.html

2 - Use Shinkansen and other trains during the day time to get from Tokyo to Sapporo. If you have the time and want to break the journey up a little I highly recommend stopping in Hakodate (Hokkaido) on the way. It is six hours from Tokyo to Hakodate and around three hours from Hakodate to Sapporo.
Hakodate:-
http://www.japan-guide.com/e/e5350.html
Hakodate access (choose to/from Tokyo and to/from Sapporo)
http://www.japan-guide.com/e/e5355.html

You could do the whole journey in one day of course.

Note that Sapporo Snow Festival is a popular place even for Japanese tourists so the trains can be quite busy during this time. When we went we even had to stand part of the way from Tokyo (to Sendai) as we could not get a reserved seat as we only booked one day in advance when we arrived in Japan.

Also, accommodation will be heavily booked so book in Sapporo in advance, or consider day tripping in from a nearby city. Also, be aware that prices may increase at many hotels during the Snow Festival especially on the weekend.

You may wish to use one of the Air Passes or alternatively there are now numerous LCC (Low Cost Carriers) available, especially if you book in advance:-
http://www.japan-guide.com/e/e2365.html
http://www.japan-guide.com/news/0020.html
http://www.japan-guide.com/e/e2364.html

Even if you just wish to fly one way this may save you a lot of time.

What is the rest of your itinerary in Japan?