Thursday is Epcot's PM extra magic hour and Friday is Magic Kingom's PM extra magic hour. ( I just looked it up, and thats the current schedule) Neither day currently has an AM extra magic hour. You probably could pick either park and be ok. Especially in January. Fantasmic is a must do for us, and if Friday is the only day you can go to to MGM... then definately do MGM.
We go at this time every January and you are going to find pretty low crowds the entire time of your stay. Saturdays at MK still get crowded. but only by January standards. We are used to rather low or no lines at the end of January. So if we have to wait 20-30 minutes for something like Haunted Mansion or Buzz Lightyear, we consider that crowded ( January crowds have definately spoiled us!!) We will be there in January 05 too., but our friends passes expire on Jan 28th so thats the day we will leave. I guess its the 18th-28th for us. Hope thats not Martin Luther King weekend!!!!!!