The characteristics which differ in P. ludlowii are size (both the plant and flowers are smaller), efarinose corolla, corolla tube is finely pilose on the inside and outside. The flowers are solitary (or two flowered) and the scape is almost obsolete. Additionally, it seems as though P. ludlowii has flowers which are darker purple in color.
|P. ludlowii (L), P. sherriffiae (R) differ in a number of characteristics|
The following definitions are often cited: Species are a group of individuals that interbreed or could interbreed in nature and thus have a high level of genetic similarity, Subspecies are groups of individuals that could interbreed but do not because of factors such as geographic isolation, but are not distinctive enough to warrant species status, and Varieties are individuals which can breed but differ from others by some consistent minor noticeable characteristics. A species may evolve over time by one or several factors such as reproductive isolation, geographic isolation, and reduction of gene flow. Two species that live in the same location may have evolved from a single ancestor species, through a process called sympatric speciation. This process is not accepted by all authorities.
Certainly further study of these plants, including a survey of variation and genetic analysis, could be helpful in determining the status of P. ludlowii, but there will likely be no right or wrong answer - just a difference of opinion. See the species gallery for more images of both P. ludlowii and P. sherriffiae.