I have noticed some of you own both the 24-105 & a version of the 24-70. What is the purpose of having both? I am looking at possibly buying a copy of the EF 24-70mm f/2.8 L II USM but I already have a EF 24-105mm f/4 L IS USM that I got in my 5DIII kit. If I were to own both the only reason I...