OBW you cut triangles to sew together forming hexagon blocks. Now there are patterns that incorporate cubes in some quilts, but the hexagon block is pretty consistent. Now, they have you sew the blocks together in rows, instead of using Y or inset seams to sew the hexagons together.
Stack and whack may have you cut square blocks, and these blocks are cut diagonally and each block only has 4 pieces. There are some with 6-8 pieces, usually you add corner pieces to square up these blocks.
There are sooooo many variations, depending on the patterns you choose.
I think the level of difficulty is about the same with both methods. Learning what a repeat is, lining them up, and to cut them very accurately are the biggest things to learn.
I prefer OBW's, but it is soley due to liking that style of quilt and I like sewing the blocks in strips versus making individual blocks :D:D:D
I think doing either of these blocks would be easier over all than the quilt in your avatar :D:D:D