They solve different route problems
Nara and Kamakura are both easy historic day trips, but they are not interchangeable. Nara belongs naturally with Kyoto or Osaka. Kamakura belongs naturally with Tokyo.
Choose Nara
Choose Nara when Kansai needs a calmer cultural day with temples, parkland and a clear historic frame.
Choose Kamakura
Choose Kamakura when the Tokyo section needs coast, air and a smaller scale after intense city days.
Compare by base and trip length
Short first trips rarely need both. Pick the one that adds contrast without adding too much travel friction.