If warmth is what you are after, I do not recommend either mainland Spain nor Portugal in December as temperatures in both countries at that time are unlikely to exceed 17ºC and will probably be much lower.
You best bet is to consider the Spanish Canary Islands which are much farther South.and average temps in December of 21ºC.
See:
http://www.manchesteronline.co.uk/travel/news/s/76/76288_az_of_worldwide_weather.html
Air Berlin operate to the Canaries from Dortmund and Tuifly from Dusseldorf and
http://www.airberlin.com/
http://www.tuifly.com
I have not been to the Canary Isalnds from many years and you would need to research this further. However, they are within the EU and I understand that depending on your destination town there is pretty much year round action.
