Kanyakumari is known as Cape comorin the lands end tip of India. A city where you can find all the three seas meet surrounded by hills, paddy fields and mountain terrain. Kanyakumari beach, Thanumalayan temple, Thiruvalluvar statue, Vivekananda Rock memorial, wax museum, Vattakotai fort are some spots to explore. Rameshwaram is one of the temple town of Tamilnadu. The pamban channel separates the land from Srilanka where you could drop into Dhanushkodi. Rameshwaram temple, Jatayu thirtham, Lakshmana thirtham, Ariyan beach, Jada thirtham, Dr.Abdul Kalam sir house, bird sanctuary are some spots to visit. Going by your question Kanyakumari would me my choice for the beaches, Triveni Sangam and piping hot Dosa.
Both places are good and costal areas. Kanyakumari is a land of devi kanyakumari and rameshwaram is a land of shri ram. Both are devotional place and located in tamilnadu. But what i think kanyakumari has more facilities than rameshwaram. Things are easily rameshwaram is like any island and surrounded from sea from all sides, so things are not easily available in rameshwaram. As per the accessibility and kanyakumari should be the first preference. Because rameshwaram is a small town. So accommodation and transportation might be a hectic task for you. You can choose as per your preference. Happy journey.. Cheers. ✌️