Very early December is great, so long as you are only there Monday through Thursday. Fridays and weekends in December are a zoo, and Candlelight Ceremony the first weekend of December is frenzied.
Be careful though as there are two weekdays in early December (the 7th and 8th this year) when Disneyland closes early at 6:00PM to host the private Walt Disney Company Christmas Parties for Disney Imagineers, employees and Cast Members from Burbank, Hollywood, Glendale and Anaheim. They close Disneyland early and then reopen it for Disney employees and their families, with Bob Iger and John Lasseter passing out candy canes on Main Street, the Disney Employee Choir performing, the Imagineering Jazz Band performing, a performance of the Christmas Parade, they pass out free Christmas Tree ornaments, and other special treats. (My neighbor works in Team Disney Anaheim and told me about the parties.)
Early December is really a mine field when it comes to picking the right day, but if you do pick the right day it can be great. By mid December it gets busy very fast, and the weeks of Christmas and New Years are always the busiest of the year.
Early May, on the other hand, is consistently easy to do. The only thing you need to worry about are seasonal refurbishments (they do these quite regularly on all the attractions at Disneyland), and possible refurbishments to Fantasmic! that might prevent it from being performed in May. Otherwise, it's delightful.
Weather-wise, you really can't go wrong year round in SoCal, except some rain in December through February.
I vote for May for a first timer.