"Fue inolvidable" would more commonly be used to describe a single experience that the person had on one particular occasion. "Era una experiencia inolvidable" is not a common usage; but the understanding that I get from the tense is that the "experience" actually occurred repeatedly as a set of individual experiences over a certain period of time. Generally, though, when you are describing a fantastic experience that you had, you are always going to express this using the preterite tense. Example: Yo tuve la oportunidad de viajar a Francia el año pasado, y fue una experiencia inolvidable.