I have done both. I did the christmas party YEARS ago and honestly don't remember a whole lot about it. I vaguely remember the snow (they used to set off snow in the streets for the Osborne Lights at the studios

another thing I am sad is gone) but I don't remember much else. the 360 fireworks (at BOTH parties) are truly amazing to behold. IF you can manage to see regular wishes before you see the holiday ones, you should see them in that order. I have seen both more than once so it doesn't matter but holiday wishes will blow regular wishes OUT OF THE WATER. I love fireworks and will see some kind of evening show every night we are there, including both regular wishes and holiday wishes... but just saying if you've never done either I would def do regular first because you may be disappointed by them after Holiday
I have a bit of a preference for
MNSSHP though. I think the after dark atmosphere is just even better for Halloween than xmas, and the addition of trick or treating and kiddos in costumes gives it the edge for me. I LOVED the parade. LOVED it. I have not seen the xmas parade in a very very long time though so I can't accurately compare right now but I have never been a huge fan of the daytime parade at WDW. It's always been "just okay" for me and for some reason in my head the christmas parade is a bit more like the daytime parade than the halloween parade is. But i haven't seen it in so long and it might be more fun at night for me. I vaguely remember a snowglobe float that took my breath away (I LOVE snowglobes) but I don't know if it's still even part of the parade. I think I also have a preference for fall and "pumpkin spice" time of year over winter and "peppermint and gingerbread" time of year.
All that said I never hesitated for a second to do
MVMCP. we had an awesome time at MNSSHP even though we left right after fireworks so I am thinking that 2 more hours of rides will make up for whatever is slightly not as great for me as MNSSHP.
If you were planning a trip and only planned to do one in your lifetime i would pick MNSSHP.
edited to add, just looked up some pics of the christmas parade and I think this one benefits from being at night too. When I went it was definitely before the castle was all lit up for christmas the way it is - I have never seen it that way and it is STUNNING. stunning. There is a shot of the parade with the snow coming down and the castle in the background and I got a little choked up. So I think I'm gonna love this one too. And I think I have to see it from main street. And I think I have to see the early parade because it comes from that direction where it is backgrounded by the castle.
@Candycane83 we have to sit right here. IMO perfect view.