Block is the easiest to use, but also the most expensive. Full sheet is the most economical but also the most inconvenient.
Watercolor paper is sold in blocks, pads, and full sheets.
A block is a pad of paper that has been glued together on all four sides. You can paint on the top sheet without removing it from the block; you don’t have to tape the sheet to a board to prevent buckling. After the painting is dry, separate it from the block using a small knife. Blocks are very convenient, but they are also the most expensive way to buy paper.
A pad is similar to a block, but the sheets have been glued together only at the top. You may need to tear off a sheet and tape it to a board before painting. Pads are less expensive than blocks.
A full sheet is a single large piece of paper that can be cut down to make a set of smaller sheets. In the US, you can buy ten 22×30-inch sheets of Arches 140 lb. for $70 (and Jerry’s Artarama almost always has it on sale).
A single 22×30-inch sheet can be cut into eighteen 5×7-inch pages:
The 22×30-inch sheet can also be cut into four 11×14-inch pages.
Or you can create six 8×10-inch sheets, plus four 5×7-inch sheets.
Full sheet is the most economical option. But storing and cutting the sheets may be inconvenient, and you may end up adding to your costs by “investing” in a large paper cutter.