Ego batteries have a limited life expectancy; maybe 200 - 280 charges and then they can be considered a disposable. Due to their slim profile, they have limitations of the liquid capacity of the juice attachments that you can use with them. It's also not recommended to use more advance juice attachments like RBA/RDA's on an Ego battery.
Having said that, Ego batteries are lightweight, more portable and pocket friendly compared to many of the larger and heavier mods. Many mod users continue to use an Ego for out of the house.
Mods are built to be more durable than an Ego battery, and can use larger replaceable batteries for longer battery time between charges. Instead of having to bring 1 or 2 additional Ego batteries with you to last all day, one mod may get you through an entire day with just a single battery or a spare carried in your pocket.
Since they are of a larger form factor, they can use larger capacity juice attachments. Mods also have a larger variety of replaceable batteries to choose from for different applications. Protected ICR, IMR high drain batteries, and the new hybrid batteries.
A mod is not considered to be a disposable device due to its better build quality and durabily. Only the batteries need be replaced, which typically are much cheaper than Ego batteries and last for 300 - 500 charges before requiring replacement.
A regulated mod will have variable voltage with a built-in ohm meter, a built-in voltage meter, and an LED display to provide useful information to the user. It will also have superior built-in protective circuitry against short circuits, over-discharge of a battery, reverse polarity, etc.
The cons to a mod are its size and weight compared to an Ego. For most mod users, the pro's far outweigh the cons.
