Xencelabs Technologies Ltd.

That really depends on your end product, the field you work in, and your workflow. 

I'll break down each of the preset color spaces that are selectable from within the Settings panel for the Pen Displays. 

Color space is truly a science and has its own language that is issued with it. I have placed a small glossary below of some of these words and phrases used with the Pen Displays. 

Color Space: A color space is a collection of specific groupings of colors supported by various physical devices. 

Gamma: This describes how smoothly black transitions to white on a digital display. A Gamma of 2.2 is considered to be standard.

Contrast: Contrast is the ratio between the luminance of the bright white and the darkest black that a monitor can produce. 

Angle of view: The angle at which an object is viewed perpendicular to the surface.

CMYK: A printer that uses Cyan, Magenta, Yellow, and black to reproduce images.

RGB: Red, Green, Blue this is used for digital images. 

Adobe RGB: Adobe RGB was developed in 1998 by Adobe to be used by CMYK printers for better Tonal output. CMYK = refers to Cyan, Magenta, Yellow, and Black. If you are creating artwork or illustrations that a CMYK printer will print, this would be a good color space for you.
sRGB: sRGB was developed in a partnership between HP and Microsoft, was the first color space created, and is still used today by most digital cameras. If you are developing artwork that will be viewed on the web, then sRGB would be a good choice for you to use.
Rec. 709: Rec. 709 is the standard camera color space for HDTV with a gamut identical to sRGB. Rec. 709 is often referred to as the standard video color space when working with video recording.
DCI P3: DCI P3 is the color space most commonly used by the film industry. Apple, Samsung, and Google also use this color space for their model devices. This is a good color space to use if you are making digital films or developing graphical interfaces for mobile devices.
REC 2020: REC 2020 is used for ultra-high-definition television. If this is an area you are working in, then REC 2020 would be a color space you would want to select.
Pantone: Pantone color space is used when you want to produce a specific color. All Pantone colors have a number guide that can be used to reproduce that color within the Pantone color space. If you are doing Illustration work or product design, this would be the color space to use.