The answer is simple: If you’re filing for only one mark, file for the word mark! Meaning in standard character format without regard to font, style, size, or color. Registration of a word mark is the broadest type of trademark protection that the U.S. Patent and Trademark Office allows. It will prevent other confusingly similar registrations no matter what design is used as part of the mark. For example:
Word mark: FACEBOOK
Why not just apply for the word mark together with a design? You can of course do that, but remember that this will require you to prove use of the exact same design over the next several years in order to maintain your registration. This may be problematic if you change how you display your design over time. You may have your brand name underneath a design but later move it to the left or right of the design, and this may be considered a material alteration requiring filing a new trademark application.
If the budget will allow for filing two applications, you can register the word mark and also the design mark, as separate filings:
Word mark: FACEBOOK
and
Design mark (b&w): 
This will protect the name and the logo, and will allow you to display them separately or together with the design on any side of the brand name and separately.
Should I apply for the design in black & white or color? If you’re filing only one design mark application, you should file in black & white. This will give you the freedom to use your logo in any colors and you will be able to prove use down the road by submitting proof of use in black & white or in any colors. If you feel strongly about protecting the colors of your logo, this should be a third application and the colors you use should be exactly the same as the colors in the application. For example:
Design mark (color ): 
We are here to help, so feel free to call us if you have questions or you are ready to file.