Do you have a preference between semi-transparent or solid (aka "opaque") stain? Semi-transparent stains show more color variations due to wood grain and changes in wood density to show through. Some people consider this to be more of a "natural" look. Semi-transparent stains are typically what is used the first time wood is stained. But with each subsequent re-staining, the color differences that arise from some areas being weathered while others still retain some color will become more exaggerated. The solutions to this are either more extensive surface prep (i.e. fully sanding the surface) or switching to a solid stain.
Solid stains have more of a painted look (uniform color, underlying color differences hidden), although the product is still a stain that soaks into the wood.
Stains come in both oil-based and waterborne acrylic varieties. The basic trade-off is that acrylic stains withstand exposure to the UV rays in sunlight better than oil-based stains do, but they dry faster and are thus a little more "brittle", meaning that they may be more prone to scratching and chipping than oil-based stains. Oil-based stains take longer to cure, so waterborne stains may be a better choice for outdoor seats and benches.
Both semi-transparent and solid stains come in a variety of colors, although "Cedar Bark" is almost universally used on new fences. Semi-transparent stains, due to their nature, do not give as much latitude for color changes because the old color will influence the look of the finished work.
Degree of Surface Preparation
Fences need little surface preparation other than power-washing and, in fact, the roughness of the wood that is typically used for fences makes sanding impractical.
For decks, powerwashing is usually sufficient to clean the wood and make it ready to accept new stain, so wood cleaners and/or conditioners are typically not needed.
If the look of semi-transparent stain is desired and the color is being changed or the deck has been re-stained previously, there may be big differences between weathered areas and areas still retaining color. In this case a full surface sanding may be necessary. Full sanding involves using power sanders to remove the top layer of wood containing the old stain color. This is a significant amount of labor and will add to the time and cost of the job.