Yes Maria  the24-70mm  is  more expensive and you wont go wrong with any of your choices the fact of the matter  that no one less will meet all your needs hence why I got two of the 3 ... 

I have been delighted with the optical performance of both the lens and in my view they run very close to the performance you would expect from primes..

As you have pointed out with the EFS 17-85 IS you own IS is somewhat curtailed owing to the f4 with the24-105 as it is with the EFS... I often use the above lens for landscape and a tripod is always best in any case.. And nearly always best for studio work too..  Having 2.8 will enable better and faster focus performance as well..  

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt