Both are billfish with pointy snouts and streamline bodies built for speed. It is the dorsal fin, pectoral fin and the bill that mark the difference.
This is a Sailfish. The dorsal fin is more pronounced, looking like a sail and the pectoral fin is larger. Also the bill is shaped thin like a rapier rather than thick like a Marlin snout.
Direct link below.
Nature - Birds and Animals, Buildings - Indoor
Links: en.wikipedia.org, en.wikipedia.org, www.google.com
By: tallturtle82