Having visited both Europa Park and Phantasialand recently I can recommend both parks as amazing places to visit. I stayed onsite so can't really advise on nearby hotels, however in terms of transport links, both are really easy to get to. Europa Park is around an hour from either Euroairport, Basel (where I flew into), or Stuttgart airport - the better flight deals do tend to be to Basel though. Phantasialand is just over 40 minutes from Cologne Airport.
As you are planning a trip in January I would check very carefully what will actually be available. Like in the UK, both parks have a winter closed season with a limited offering over the Christmas period, therefore you would not be able to access the full park at that time of year. Phantasialand is allot smaller than Europa Park, if it is your first visit to a German theme park I would probably lean towards Europa, however whichever one you visit I know you'll have an amazing time