Coins or pieces of gold, silver or other previous metals and stones are probably easier to transport, or trade (person to person), but often come at a small cost. You also need somewhere safe to keep them. American Eagles, British Gold Britanias, Canadian Maple Leaf, South African Kruger are some of the better known coins, but watch out for the true gold content, and potential forgeries (if buying privately etc).
Paper / Electronic Certs are very convenient, really bought online, and usually give comfort that your investment is securely held (at The Perth Mint, for example).. while they can be bought and sold quickly, that relies on electronic trading being available, the location, where your assets are held, remaining safe etc.