For Giclee prints,always work in RGB or CIELab: never in CMYK. CMYK is in no way 'more professional' or 'better' than RGB or Lab.
You may wish to use CMYK for our Digital Print services. Details here.
CMYK is a device-dependent colour space, intended for 4-colour print systems such as offset litho. Furthermore, CMYK uses four channels to describe an image rather than three, and is therefore not as efficient.
CIELab is completely device-independent.
Some RGB spaces are device-dependent (for image scanners and display monitors), and others are independent, such as AdobeRGB and sRGB.
We'd recommend that you select AdobeRGB as your default RGB space.