Both treadmills have built-in speakers that allow you to plug in an MP3 player (such as an iPod), and listen to your music out loud, keeping your workout headphone wire-free. Both treadmills have a red LED readout that shows the speed, incline, time, distance traveled, calories, pulse and pace you are at. The S77 is a step up from the lower cost S73 in that it has a 3.5HP motor (as opposed to the S73's 3.0HP motor), as well as a 400 pound user weight limit (the S73's is only 350 pounds). However, the S77 lacks the higher end TT8's 425 pound user weight, 22" x 60" running surface, 32" x 80" deck (the S77's is 30" x 80") and 3 inch rollers (the S77's are 2.75 inches). The Sole S77 comes with a lifetime motor, deck and frame warranty, five year parts warranty and two year labor warranty.