Captains only in our company, and I'm quite happy with that. A low experience FO is quite capable of calling the failure but is not, in my opinion, equipped to make the reject decision. It's a mighty big one and a wrong call could lead to total loss of the airframe and possibly worse. Of course it's a different matter with higher time guys but the SOP has to be written to suit everyone. Other operators do it differently but what we do suits me fine.