Luke seems to describe Christ's ascension on the same day as his ressurection (Luke 24:13-51), whereas Acts states clearly it was forty days after (Acts 1:3ff). What do you think?

Yes, at first blush it does appear that the ascension and resurrection took place on the same day! But keeping in mind that Luke authored both Luke and Acts -- where the ascension took place 40 days after the resurrection -- we must give him the benefit of the doubt. It is quite common in scripture for events to be telescoped. That is, events are mentioned in the same breath that may take place days, years, or even centuries apart.

Luke ends with the ascension perhaps because this is how he is beginning the book of Acts; the ascension is the transition between Volumes I and II of his magnum opus.