I like the 30-square board. You can have a one-row puzzle filling the entirety of the board, a two-row puzzle (each row of the rebus fits nicely in a block of 15 squares) or a three-row puzzle (each row being 10 squares), with no overlap into another row, i.e., there won't be something dangling from the first row of a 3-row rebus behind square 11. It seems Concentration has less empty squares than Classic, particularly in the corners.