Consider there are two wheel types of ab rollers that offer stability. Those with foam-covered handles are more comfortable. That seems to be important.
There are different types. Some have two wheels and offer extra stabilization. Others have one wheel.
Consider differences in construction. Is the handle made of steel or plastic? If it is made of plastic, what is the gauge or thickness?
Is the grip textured or smooth?
Consider comfort: Is the grip padded?
Consider durability. Will it break during heavy or intense use?
Some models have foam-covered handles. Other models have handles made of rubber.
Some models have adjustable handles. Others can be disassembled when not in use.
Consider capacity – how much weight can the roller handle?