I currently have a T2i and want to purchase a 6D. My 17-55mm is my favorite lens for the T2i but it will not work on the 6D. It looks like the 24-70mm will be very close to the 17-55 (27-88mm) but which one do I get? I really don't want to spend the $2,000 (give or take) for the 24-70mm f/2.8L II USM so I'm left with the EF 24-70mm f/4L IS USM ($1,200 used) or the a EF 24-70mm f/2.8 USM (used for about $1,000). I would consider the II USM if everyone believes that it is the only option but I can't see how I could justify a $1,000 difference in price. And while you're at it, is there really that much difference between the two two f/2.8's? Thanks.