You can click on the airplane in the itinerary and it takes you to the seat options, with full descriptions of Prestige and First.

Typically, the First Class seat is a few inches wider and longer, but the real difference is a privacy door/shield. Again, there are clear pictures of this on the KA website.

Unless you are concerned about people snapping your picture and posting it to facebook, save the miles, Prestige is just fine, and typically with lay flat seating.