I think everyone is different and which one you like best depends on the person in particular.
We went to both parties in 2005 (MNSSHP in Oct and MVMCP in Dec)
Both are great!
I though that while both parks were decorated, the Christmas decorations were much more impressive, such as, HUGE Christmas tree and Main street was decked out! Both have great character costumes.
I just love Christmas time and found it to be more enjoyable for me.
I think the MNSSHP was something where I found it to be really cool, while the MVMCP was a more "fun" and "happy" atmosphere.
My personal opinion - Holiday Wishes (or FITS as they call it) BLEW AWAY THE Hallowishes at MNSSHP. The musical score and display were just so much more impressive to me.
Both parades are really cool but I liked MVMCP better.
A big factor to me is that it was nice to be able to be in the MK when it was a little brisk out, and eating free cookies and hot cocoa (all you can eat/drink).
A VERY VERY cool thing also is the "fake snow". the snow is really cool on Main Street. With the music, the lights, the decorations, the tree, and the snow, it is just a truly magical experience. You dont want to leave.
Also, if you go during the Christmas holidays, the other parks are much more decorated for Christmas than Halloween.
Both are very cool, dont get me wrong, but, I just found so much more enjoyment at MVMCP.