The difference is quite simple: Maris Otter is a type of barley, Hugh Baird is a brand.
Maris Otter is a two-row winter barley and works well in any English beer style. It's available from several different maltsters.
Hugh Baird & Sons is one such maltster; they produce malt from two-row UK barley at facilities in Pencaitland, Scotland and Witham, England. One of the malt varieties that they offer is Maris Otter Pale Ale Malt.