On such a short visit I'd go for Shatti al Qurum or Mutrah.

Chalk and cheese really. Both delightful in their own way, and both on the coast. Shatti is a smart beach and residential area, even boasting the new Muscat opera house. Mutrah is the old port and commercial zone, including a vibrant souk.

Shatti has an IC and a Crowne Plaza. The IC is the better located. Both are pricey, but there are other, less expensive places.

Muttrah has non-starry, non-chain, hotels and guest houses. Look on trip advisor.

Ruwi is the inland business and commercial part of the city. Can't see any tourism joy staying there.

One consideration might be transport. Shatti is closer to the airport; but if you are uncluttered with bags there's a bus from (close by) the airport that will carry you to both areas.