A comparison of ANL and MEGA fuses

The ANL fuse has a window where you can see if it is blown or not, and the contacts are plated to reduce corrosion, and in addition the ANL fuse is ignition protected so it can be used in an engine compartment of a gasoline powered boat. The Mega fuse produces a spark when it blows so isn't suitable for use near gasoline. Mega fuses are available up to 300 Amps but the ANL fuse comes in sizes all the way up to 750 Amps

One of the big differences is Interrupt rating.  Interrupt rating refers to the level of current that a fuse can safely interrupt.  In severe overcurrent situations a fuse will blow but vaporize and the metal will condense right back on the base it has just vaporized from.  The end result is that it doesn't interrupt the circuit and the fault continues despite the fact that the fuse has blown.  ANL fuses have an interrupt rating of 5000 Amps which by the ABYC standards allows them to be used as the primary circuit protection for large battery banks.  Mega fuses have only an interrupt rating of 2000 Amps and cant be used as primary protection on large battery banks

As with most things, you get what you pay for !