Both are 60’s U.K. bands, both did the concept albums and the "progressive" rock thing. Yet, they are so different.
I’ve always found the Moody Blues to be happier, more upbeat and positive. Whereas Pink Floyd are just about the opposite. Also, Floyd has more famous albums I believe and are acknowledged by the Rock & Roll Hollow flamer types.
In the end, I’d take the Moody Blues to that “desert island” over Pink Floyd in a heartbeat. I think I'd want their upbeat brand of rock.