There are differences between these two options. The conversion option usually uses the mechanical tuning mechanism of the old radio and I would think provide an input for an mp3 player. The conversions usually provide both AM and FM.
The completely new models provide AM/FM and MP3 input. The installed radio looks pretty well identical. I think the buttons give electronic tuning which is faster, more accurate and more convenient than the old mechanism. The radio chassis is smaller, lighter and saves space behind the dash.
