Ok, have done some more research and called down there. Am posting this for any members with similar inquiries in the future, and changing the title to include Spicebush...
Swallowtail units are larger, with a Master Suite upstairs and the second bedroom downstairs off the living/dining/kitchen area. The Master Suite has a separate seating area that can be used as a third bedroom. The master bath has a large soaking tub.
Spicebush units have both bedrooms upstairs, and lack the extra sitting area. Spicebush kitchens have been recently redone, Swallowtail kitchens are older.
Spicebush is a short bike ride or long (too long for us) walk to the beach, but the trolley doesn't stop at Spicebush. Swallowtail is across from the Sea Pines Center, with a few shops and the trolley stop. Swallowtail is closer to Harbour Town, but it is a long walk (again, too long for us).
Swallowtail at Sea Pines
Spicebush at Sea Pines