In general foam core vs built up are very close in weight when the foam core wing is properly built. But too much glue and/or heavy wood can quickly cause the Foam core wing to gain alot of unwanted weight. The sheet attachment methods vary with contact cement or very thin layer epoxy being the traditional methods. The old D&B kits use a almost totally sheeted wing meaning that the foam core was cut with the leading edge. So you have to sheet around the leading edge which could be somewhat challenging. Traditionally you just sheet the core, glue on a balsa leading edge and whatever it takes for the trailing. Cutting foam cores is not a real hard thing to do.