A wood core is generally livelier and more durable than a foam core, although you won't feel much difference in performance. Both cost about the same. This is determined by the thickness of the core and by the quality and direction of the fiberglass layers. Torsional flex is the amount of twisting a board will allow along its center axis. Stiffer boards are more stable at higher speeds and hold an edge better on very hard snow.
Softer boards are easier to turn and therefore better for new riders.
With cap construction the topsheet extends to the edges. With sandwich construction there's a vertical sidewall between the edge and the flat top surface. There's no significant performance difference between the two. For recreational riding, the board's tip should reach somewhere between your collarbone and nose when you stand it on its tail. It should be just wide enough that your toes and heels won't overhang the edges.