I have been to both (2 times for each one) and would have to say hands down WDW. Here is why:
Candlelight Processional at Epcot: One of the most beautiful things I have seen. Do the dinner package! It doesn't reserve you a seat but rather a place in line before the non dinner pkg guests many of whom will not get in.
Illuminations: At the holidays, Illuminatios has an extended version with its own finale and its wonderful. A bigger and better twist on an already spectacular show.
Epcot countries have their own Santa's and traditions respresented which is very fun to see and experience. And you can do some nice holiday shopping too.
Osborne Lights: This is at DHS and its wonderful. I could walk around for hours and look at that.
Resorts: All the resorts decorate, the monorail ones in particular. Its fun to just ride around and enjoy the ones on the monorail but they are all really pretty. The Epcot resorts and AK also have lovely decorations. There are threads on the WDW Resorts Board with photos of the various resorts. I think my three favorites are AKL, GF and WL. GCH has lovely decorations too on par with WL, but the DLH while very lovely wasn't as decorated as I had hoped and PP was even less so. If you stay at a moderate, CSR has the best decoratons in my opinion in the lobby area. CBR and POFQ were both lovely when we stayed at those, but nothing like the pictures I have seen of CSR. The values also have really pretty trees in the lobby, so regardless of what category you stay in, you'll have something festive and pretty to enjoy.
Mickey's Very Merry Christmas Party: This is a very fun party, hard ticket event and its wonderful. Its not cheap like all things Disney but its packed with holiday magic, parade, special fireworks (that are incredible) and you get cookies and hot chocolate too. Note, that the parade and fireworks will be shown to all MK guests once Christmas week itself arrives. Before that if you go earlier, the party is the only way to see those two things.
Mickey's Jammin Jingle Jungle Parade: I enjoy this parade all the time, but its very cute and fun during the holidays. I don't know why, but I got such a kick out of this thing a few years ago. I had to sit there and watch it like three times the ten days we were there!
All four parks have lovely decorations and their own theme. DL and DCA are also wonderful and I thoroughly enjoyed our time there.
Disneyland itself was lovely. But once I did WDW at the holidays, I came to realize that is in a league of its own. I do think if you can, go for at least five days to have time for each park, the party at MK and some time to explore the resorts.
WDW really pulls out all the stops, I haven't been to DL during the holidays in four years though so they may have addes some things since my last stay. Again, it was wonderful, but WDW was just incredible. They have so much more room to just decorate and do special things. I think Epcot in particular is just incredible. This archway of lights as you walk back towards the World Showcase. We did our family Chirstmas photo under those lights a few years ago.