If you are preparing documents solely for the web, then for the same number of image samples RGB is negligibly faster with possibly smaller file size. If you are preparing documents for print, then CMYK is better since it preserves spot colors, and separating RGB may be unsatisfying or not possible. If you are preparing documents for both web and print CMYK is the better choice. CMYK documents coexist quite nicely in the RGB world, but the converse does not always hold true. And there are ways to convert high-resolution PDF images into low-res for the web.
There are cases though where you cant always choose CMYK. Many PDF documents originally created using Microsoft or other PC applications will arrive in your shop as RGB. Your choices are basically two: 1) immediately convert them to CMYK, using any of several 3rd party tools for Acrobat, or 2) rely on downstream processes, such as in-RIP separations, to convert RGB to CMYK.
Monitors, scanners, digital cameras, etc. deal in visible light. The color spaces to express these (RGB or various ICC color spaces) are designed to include all visible light. Printing, on the other hand, deals in light reflected off or through inks, which allow a narrower color range, or color gamut. An on-going issue is how are more visible colors converted into the fewer colors of the CMYK print gamut. See the Rendering Intent table above for more on this subject.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.