Depends on the specialty and the geographic region but either is OK. I was fortunate enough to be able to take both months off for interviews. The downside of December is that nobody will be interviewing from (roughly) 12/22 through the end of the month (everybody takes the Xmas --> NY week off) so that means you really only have 3 weeks of interviews. The downside of January is that the weather sucks and you may have trouble with travel.
Whichever one you choose though, be sure that the second you get an email offer for an interview, you write back to request a date. If you miss out on the choice interview dates, it may not matter which month you take off. This is even more important if you're applying to a smaller specialty (other than IM/FM/Peds/Surg) where they may just have 1 or 2 interview days a month.
