Woz' interview in Founders At Work seems to confirm that Jobs played a major part in many design decisions on the initial Macintosh, especially the removal of extension slots:
We had a real argument over slots. (...) Steve said, "All people really need is a printer and a modem." And that was just false because he'd come from a different world than I. (...) He wanted just one slot for a printer and one for a modem(...), so I said, "If you want 2 slots, get another computer." That was the only time we had a real argument.
We had a real argument over slots. (...) Steve said, "All people really need is a printer and a modem." And that was just false because he'd come from a different world than I. (...) He wanted just one slot for a printer and one for a modem(...), so I said, "If you want 2 slots, get another computer." That was the only time we had a real argument.
http://www.foundersatwork.com/steve-wozniak.html