This question is music to a plant breeder's ears. It shows that the producer is aware that cultivar choice is important to the success of a farming or ranching operation. The first step in deciding which variety to plant is to decide which species to plant. A good starting point is to first determine what you, the producer, need to accomplish. Is the need for warm season forage? Winter pasture? Pasture or hay? Wildlife habitat plus forage production? Do you want an annual or a perennial? Is ease of establishment important, which is to say, can you afford to lose most or all of a season of production to allow the stand to become established.
Do you need maximum tons per acre, or is premium quality forage the main concern? Do you want nitrogen fixation? Once the objective has been defined, a person should then determine what resources they have. What soil types are present? What's the soil fertility (and are you willing and able to correct it if need be)? What other forage resources do you have? Equipment. Irrigation? Financial resources?
Finally, ask yourself whether you really need to plant, or if you can change management practices and use currently available forage resources to meet your goals.
Once you have decided to make a planting, the next step is to gather information on species and varieties that can meet your needs and fit your resources. There are several sources of such information, including university extension personnel, Noble Research Institute specialists, seed company personnel, and fellow producers.
If you have access to the Internet, the Forage Information System site (extension.oregonstate.edu/coos/Forage/index.html) has plenty of information on various species and varieties of grasses and legumes. After choosing one or more species, decide which variety(s) to plant. I suggest resisting the temptation of planting a variety because "it's what was available" or "the seed was the cheapest."
Sure, seed availability and cost are important, and a hard-to-find, expensive variety may not necessarily be "the best" variety. But in the case of a perennial, a pasture might be used for decades. Thus, a few dollars more for seed at planting time may be a great investment over the years if Variety X performs better compared to "cheaper, available" Variety Y. The point is, it often pays to gather a great deal of information and shop around. Finally, remember that there usually isn't any one variety of a given species that is "the best." It depends on your goals and resources.