Boxlock is the OLDER of the two, patented in 1875 while the Purdey sidelock came along in 1880 and the Holland in 1893.The boxlock introduced barrel cocking that made the sidelock possible.
From an engineering point of view, boxlock is the better action. I would opt for the ones with stock bolt, becaue I can pull the stock and inspect and service the action.
The best engineered boxlock action is the Beretta 626. It beats the Anson Deeley on all points. Beats the sidelock too, so much so that a a comparison would be unfair. But these are thoughts for those that appreciate good engineering, not for collectors.
