- Screen orientation [4th gen is more wide screen in design, while the 3rd gen is full screen]
- Battery life, the 3rd gen has a better video playback life, but it is also an older model which means the battery probably wouldn't last as long
- I like the 3rd gen interface better in my opinion, but this is all a matter of preference.
- Album art is larger on the 4th gen
- Quick shortcuts available on the 4th gen, not available on the 3rd gen. You can skip straight to the album or artist by holding the center button.
- Head phone with remote support for the 4th gen, not available for the 3rd.
- 16GB capacity available in the 4th gen, only 8 for 3rd gen...but this probably wouldn't be a issue if you are only using it for exercise.
I voted 4th gen, but that is simply because a) I've been accustomed to the bubble gum design since I've owned 1G, 2G, 4G. b) The color options are far more modern, the 3rd gen just makes you feel like you are in the 80s.