It often comes down to what you can get your hands on, these dwarf species aren't always easy to get a hold of, depending where you live. It's also a choice of preference. I like Pelvicachromis (krib) varieties best, but many others prefer Rams or Apistogramma. It's best to do some reading up on all of them before making a choice. Rams are, as a generalisation, most delicate, while Kribs are the least.