Many corgis do have strong herding instinct. They will chase poultry and sometimes they'll chase other small animals. It is possible to keep them separated though, and depending on the small animals it may be possible to teach the dog to ignore him. Keep in mind small animals are not stock to be herded and part of herding instinct includes species discrimination, but that doesn't always work.
Depending on the small animals, corgis might kill. Corgis do have a long history of being ratters. They also very well might hurt/kill poultry if left to their own.
As for which is better ~ I very much prefer the pemmies.
It would be helpful to know a bit more. What are the "small animals"? Are you going to use the corgi to herd the poultry? What are you looking for in a dog? Energy levels, working abilities, temperament?
Pemmies are usually more outgoing with strangers than cardis, and often are more athletic. I haven't seen a lot of working quality cardis ~ or even sport quality, but I have seen a number of working pemmies. Pemmies are often medium-high energy and drive.