I created both the DISboards' "Halloween at DLR Superthread" and the "Disneyland at Christmas Time Superthread" specifically because I love both holidays so much and wanted to share that love, as well as lots of good information, with other DIS'ers. I really appreciate that DLR is beginning to kind of flesh out its Halloween offerings a bit more as the years pass, and I hope that Halloween Time as a season only begins to get bigger and better because it really is quite festive and fun.
However....that said, there is absolutely no question in my mind that Christmas time is the time to go. Halloween Time only scratches the surface at DLR - none of the hotels are decorated, Downtown Disney is not decorated, many of the shops and restaurants are left with no traces of Halloween at all, and California Adventure is all but ignored (with the exception of a Duffy Halloween photo spot).
Christmas time at Disneyland Resort is soooo much more extensive, thorough and detailed.
For example, the Christmas trees at the Paradise Pier Hotel and the Grand Californian Hotel are both gorgeous in their own way - they should definitely be seen. All of the shops and restaurants all over the Resort have decorations and/or trees. Downtown Disney is alive and lively with holiday music and its own giant tree. Santa's Reindeer Round-Up is a gem in Disneyland, full of charming, rustic details. The window displays on Main Street are adorable. There is now a gingerbread smell that fills the air when the snow falls on Main Street. It's a Small World Holiday is stunning to see when the colored lights are on at night, and the entire ride is transformed inside the brilliantly lit facade. Toontown has its own wacky style of decorations. New Orleans Square is exquisitely embellished with Mardi Gras-influenced beads and masks. The nighttime Winter Castle is aglow in sparkling icicle lights. A Bug's Land in California Adventure has giant lights and ornaments. I could go on... It's much more of a feast for the senses than Halloween Time is.
As AmericanItGirl said - and this is exactly what I have said, word for word, for years too - you get much more "bang for your buck" with a DLR holiday trip.
So absolutely, hands down, no question - unless you simply prefer Halloween over Christmas as a holiday (and a lot of people do, so that would be understandable) - if you can only pick one time to go to DLR, choose Christmas time. The beauty of that season at DLR is unmatched!
Oh - and as for weather - I'm a SoCal resident and have seen December weather get as hot as 90 degrees and as cold as 30 degrees over the years. It has been in the 60's for most of the days recently (a contrast to the warmer temperatures last year), and in the 40's or even 30's at night, for the most part.