Hospital (SGH) first since they're government and usually have the full range of ultrasound services, or try the private ones like Normah Medical Specialist Centre and Columbia Asia which a few of my mum friends used for their detailed anatomy scans. I'd suggest ringing ahead to confirm they do the 20-week detailed scan (not all clinics offer the full package), and ask about waiting time and appointment slots since these places get quite busy!
Ahhh detail scan in Kuching, honestly you'd want to check with the private hospitals like Sarawak Medical Centre or Normah Medical Specialist Centre, they usually have the better ultrasound machines for detailed scans. One of my mum friends did hers at SMC and said the sonographer was really thorough, tapi price lah a bit more mahal compared to government clinics, but worth it if you want yang clearer images!