I personally liked Halloween a lot more. Christmas was beautiful but more of a mellow event. The decorations for Halloween were extraordinary, and the parade was wonderfully catchy and mind-blowing. Halloween was a very up-beat event.
Christmas of course still has great decorations, a gigantic Christmas tree (I mean ginormous) and a parade, but IMO not as fantastically put together as Halloween.
Christmas in Japan is definitely not going to feel the same as North America. As mentioned, it is not a family holiday but a couple's one (like Valentine's Day, and even THAT is another story). It's not very big of a deal, but their New Year's is more like a Western Christmas.
So the big thing to do on Christmas in Japan are:
1) Eat KFC.
Because you can't get turkey in Japan, so the next best thing is KFC (and boy, they have an uncanny love for Colonel Sanders). They even dress up the Colonel in a Santa suit. Seriously, watch out for it! And it's so popular that you have to pre-order your meal to guarantee your chickens by Christmas.
2) Christmas cake
Because of "fruit cake". Christmas cake is usually vanilla cake with white frosting and strawberries, but really anything goes. Families eat the cake together or people can host an Xmas party with friends and have cake there and eat it too.
3) Couple Date Night
It's a snowy day for love. Snow is considered to be utterly romantic just as a moonlit waterfall in an enclave is. Couples exchange presents and go on dates. If anything, it feels a lot more like V-day than Christmas.
Also keep in mind that Christmas is not a national holiday. Some people are even working on this day and shops are open.
Since you are going to Christmas, it will still be wonderful! The next time you get the chance, definitely check out Halloween, too!