Pieces of vinyl and aluminum siding are flexible. Vinyl in particular can look uneven, distorted or wavy on the house if the installer doesn't take extreme care. Aluminum is more rigid. Choose the color you want from the siding samples themselves, as colors in catalogs may not be accurate. Siding comes in various profiles with unfamiliar names.
Additional profiles match other horizontal and vertical wood siding.
New vinyl or aluminum siding can be nailed directly to the existing walls, if the existing material is sound. That's when you're apt to encounter unexpected costs for structural repairs, weatherproofing or even asbestos-shingle removal. You may want to consider having the contractor install rigid foam board first, before putting up the new siding. You can relocate or enlarge existing openings with little added disruption. A knowledgeable contractor will also be able to set the new windows so their moldings extend adequately beyond the new siding.