I would agree that GHB is a bit of a cleaner feeling in the sense that it doesn't knock your ass down real quick and run waves of madness over you for a while. I personally prefer GBL for that reason. It's more of an ass kicker and honestly has given me more euphoria in most situations, while not making me as tired as GHB once it wears off.
I never noticed GBL lasting longer than GHB. In fact for me I'd say it's the opposite. GHB is about 2-3 hours on a strong, single dose, while GBL is probably 1.5 hours max. It's hard to tell though, since I don't know too many people who do a single dose of G without re-dosing over and over throughout the day or night. That's just the way taking caps is for most people I know who have experience with it.
So to answer your question - it's definitely not true. They are about the same duration, but it varies from person to person.