"How do the two ivory shades compare?"
While the photo isn't overly clear in my opinion revlon colorstay - ivory is a much lighter pink undertoned foundation. The rimmel lasting finish - ivory is a darker colour and is yellow undertoned.
Swatches of Rimmel 25h Lasting Finish in five shades: here
Review of Rimmel 25h Lasting Finish Foundation: here
Tutorial on how to make your Revlon colorstay in to a pump action foundation: here