Lens 'overlap'

I'm new to the forum and hoping for some good advice.  I currently have a T3i and I'm about to upgrade to a 6D which will require I also upgrade my 'everyday' lenses.  The lenses I will need to replace are my 18-55mm f/3.5-5.6 IS II 'kit' lens and my 55-250mm f/4-5.6 IS II zoom.  I have already decided on the EF 24-105mm f/4L IS USM lens and the EF 70-300mm f/4-5.6L IS USM lenses.  I'm concerned I won't be satisfied with 24mm for wide-angle shots, so I'm considering also purchasing a wider angle lens and have my eye on the EF 17-40mm f/4L USM.  My question is: how much difference will I notice between 17mm and 24mm at the wide end?  I realize these lenses overlap some, but my concern is gaining a wider angle.  I've looked online at the various wide-angle lenses - both fixed and zoom - and the price difference is huge!  Still, I don't want to drop $700 on the 17-40 if there isn't a noticeable advantage over the 24-105 for landscape shots.

Thank you!