Transparencies make better reproductions than photos printed from negatives and larger reproductions require larger transparencies. If a 35mm slide is enlarged too much, the image is degraded. For magazine or book illustration, 4x5 is adequate, but for art print reproduction, 8x10 is preferable for larger-size prints. Making a 4x5 enlargement of a 35mm slide is not acceptable for reproduction. These are often lower in height than the usual drafting table surface or easel and can serve as a convenient surface to place your palette on.
Hot press is very smooth, rough is highly textured, and cold press is somewhere between the two.
Cold press is probably the most commonly used because watercolors are harder to control on smooth paper and rough tends to create more texture than many artists prefer. Paper surfaces will differ greatly from one manufacturer to another, so you may find a cold press that seems more like hot press or a rough that is extremely heavily textured. The side you like best. However, most paper surfaces are different front and back, so choose whichever you prefer. I know of two methods.