There is a chance that the Star Wars Half Marathon will be held from Jan 12 to 15 in 2017 (it was over the MLK weekend this year). If that happens, it does mean the crowds in the park will pick-up starting on Thursday and stay fairly high through Sunday. It might not mean super long wait times, but it usually does mean a more crowded feeling in general. Also, in January, a fair number of attractions often go down right after the holidays official end. Most likely Haunted Mansion will be closed (getting its holiday overlay removed). This year (2016) has been different, but it's worth noting that historically during the non-peak season, DLR usually have shorter operating hours and reduced entertainment (e.g., fireworks are normally only presented on weekends).
Easter is mid-April next year, so you'll probably be getting there just before the peak of Spring Break crowds (usually the week before and after Easter, but with Easter being so late, it might be the two weeks before Easter in 2017). Historically, it means longer operating hours and more entertainment. Refurbishments are hit-or-miss (there aren't any well known seasonal overlays that are consistently added or removed during this time).
Personally, given that you aren't going that close to Easter and you are planning on going on weekdays (W/R), I would go with the March dates.