I haven't been to Mauritius yet, but lived in Oman for almost 10 years.

The weather in January is perfect, although evenings may be cool and cloud and (believe it or not) heavy rain are possibilities (check out last weeks rainstorms in the UAE). I'd recommend avoiding May-September when the temperature and humidity can be unbearable and the holy month of Ramadhan when alcohol sales are severly limited, bars are closed and most restaurants are closed in daylight hours.

Best hotels are the Chedi, the Shangri-La resort and the Al Bustan Palace (owned by Intercontinental). The Grand Hyatt and Muscat Intercontinental are also good choices.

The country itself is beautiful and retains much of its old-world charm and is very safe. There's plenty to see and I'd recommend including a 4x4 trip into the mountains of Jebal Akhdar and a trip into the Wahiba Sands (stay one night in a desert camp). Oh and the scuba diving is very good.