The 757 and 767 are basically the same plane, though the 767 is bigger (wider, with two aisles down the plane instead of 1). They have the same cockpit controls and are flown by the same pilots. For DL, the two are interchangeable within the fleet for longer domestic flights. Since the autumn months are generally less busy for leisure travel than the summer, some routes that use the 767 may be "downgauged" to the 757, or 757 to the smaller 737, and so on.

In your case, the domestic version of the 767 has identical seats and entertainment to the 757 you'll be flying. Some people feel the 757 is more intimate because it's a single-aisle narrowbody; some feel the 767 feels more spacious, since it's a widebody. Many prefer the 757, since boarding generally uses a door behind the F cabin. Therefore, you don't get 200 people tromping past you on their way to coach watching you sip your pre-departure bloody mary. On the 767, the door is at the very front of the plane.