I have a new gas dryer to replace an older one. The older unit was connected by black pipe from the inlet on the appliance to the gas supply (natural). There was a shutoff and a union. My question: Is it better to use a flexible type connecting pipe from the shutoff to the inlet or use 1/2" black pipe all the way.

Go the flexible. It's much easier, cheaper, and allows you to move the dryer if and when necessary. I'm surprised to hear that it used to be solid pipe all the way. I've never seen one done that way.

PS: Of course you'll still need the shutoff between the black pipe and the flex.

Go with the flexable but make sure it has a ring on it telling you that it is approved by the AGA, American Gas Association