It is all a matter of of how much space you may have available wherever you put the banner. If you use it as a widget in your sidebar, then be sure to check it after entering your banner information, as it may throw all your information off kilter and make the site look messy! I do not think the color will matter too much as there are too many people that like different colors, so just go with what "you" like and maybe change it out every now and then to something different.
