They are very similar but I think there is a subtle difference. If I see an "inspirational" performance by a person, then I feel amazed at that person's achievement. The focus is more on the other. If I find someone or something "inspiring", as a result, I want to be a better person or I am drawn to contemplate the deeper realities of life.

I can't think of any difference; I think they're the same.

Not much. It seems to me that "inspiring" refers to the EFFECT, "inspirational" to the INTENTION. "He had one of those inspirational posters on the wall. I thought it was silly." The poster was supposed to be inspiring, but it wasn't. "It was just a seagull looking for garbage, but I found its graceful flight inspiring." The seagull wasn't trying to be inspiring, but it was.