Only get it for these reason...
I would only get the 2.6 for these reasons:
If you are editing high definition footage, over 30 or 60 minutes in length.
If you are using Motion for HD content.
If you plan on reselling the computer in the future it will sell for a little higher being 2.6 than 2.5
The only real difference in speed you'll see if rendering HD content over 30 or 60 minutes in length. It will cut your render times by a few minutes which CAN save you on a deadline or if you are working in the multimedia business with clients.