Wintertraining - Cross,-or Montainbike?

Hi everyone!

Next year I'm participating my first Ironman in July here in Germany. The last years I came over the winter with one or two hours a week on my hometrainer, but I''m afraid that will not be enough to prepare for an Ironman. So I ask myself, what kind of bike would be the most suitable to keep my endurance up over the winter. You know, I live in northern Germany, the landscape is predominantely flat and only a few small hills. Here are two kinds of bikes:

http://cgi.ebay.de/MOUNTAINBIKE-MTB-ALTERO-2-0-FAHRRAD-VK-FREI-10142-26_W0QQitemZ320409406962QQcmdZViewItemQQptZSport_Radsport_Fahrr%C3%A4der?hash=item4a99e38df2&_trksid=p3286.c0.m14

http://www.stevensbikes.de/2008/index.php?lang=de_DE&bik_id=380


Which one do you recommend? I think the fun-factor is higher with a MTB, but the terrain is not really made for mountainbiking here....

I hope you can give me some advice, the fall is coming with a full head of steam and I will not be able to use my Tri-bike on the road in in three or four weeks any longer (falling temperatures, rain, storm, wet and slippery roads ).

Thank you very much for answers!

Best wishes!

Timo