Which one is a better show???
I don't know that I would really say either show was "better" - they different from each other. I think it really depends on whether you like Frozen and the holidays or wanted something more movies/
Disneyland specific (but still with Frozen).
I've seen both shows and neither really "wow'd" me in the sense that I'd make a huge of point of going back to see either show. They were fun to see, but the viewing area is still tough to deal with (it's not friendly to children or shorter adults). You really have to be willing to get there before they open the viewing areas so you can grab a rail spot if you want to see the show as it was meant to be seen. If you can't see the water surface, it's basically just mist screen images with dancing water fountains.
Winter Dreams was fun, but you have to like Frozen. You have to remember it was developed during the height of the Frozen-craze last year, so it features Frozen (especially Olaf) pretty heavily. Other than that, it was a cute show featuring holiday images and music.
I saw WoC: Celebrate about a month ago and I'm of mixed feelings about the show. Of the three new shows that debuted for the 60th Anniversary, I do feel that WoC: Celebrate is the only one that is really about Disneyland. It's not entirely about Disneyland (they still worked in Let It Go and Frozen, along with clips from other animated movies), but it does feature Disneyland. Whereas both PtN and Disneyland: Forever really aren't about Disneyland - they're about Disney and Pixar movies. So in that way, I did like WoC: Celebrate, but I did think it was a bit slow in parts...it felt like it had uneven pacing. I'm also on the fence about Neil Patrick Harris' performance; there was nothing wrong with it per se, but his voice (singing) wasn't as strong as some of the others in the show and I felt like that detracted from the overall show.
If you'll be back at some point when it's not the holidays, I would go see WoC: Winter Dreams. If this is your only trip (in the near future), I would just go to whichever show works into your schedule better.