The ones you linked to are some low budget watercraft, and I'm not sure you will find any 6" SUP in that price range. I looked quite a bit at them last spring and I think you'll have to pay at least $500 for one with decent rigidity. Tower and Saturn make some that looked good to me. Sea Eagle's SUPs are even more money.
In the end, I saved myself the cost of a SUP. A local company that sells and rents SUPs also holds a paddleboarding class on a nearby small lake every Saturday. For $20 I joined the class and got to paddle a rigid, 32" wide SUP with the group for about 1.5 hours. I discovered that it is
work. You constantly are working those leg muscles in the battle to maintain balance. It was hard to appreciate and enjoy the beauty of the day or of the lake because I was too busy concentrating on not taking a bath. And paddling really works the abdominals, too. My legs were so sore by the end of class, I knew I did not want to own a paddleboard. Instead, I only bought an inflatable kayak.
Mike G.
Liberty is meaningless where the right to utter one's thoughts and opinions has ceased to exist. That, of all rights, is the dread of tyrants. --Frederick Douglass
photo: Yosemite Valley view from Taft Point