I've never been to The Big Island, but have visited family members there several times.
Guam has no volcanoes, no Hawaiian hospitality, no black sand beaches. It does have, however, some of the nicest snorkeling beaches, its own brand of Island hospitality (which is a lot more laid back than the Hawaiian kind), and some bargain shopping. Also, the beaches at the southern end of island are just gorgeous.
If you are currently living in the Asia, Guam will be a good place. I would not go all the way there just to vacation if you live in Mainland US. It is too long a flight.