This is going to be a quick flyover of the pros and cons of different aspects of saline breast implants. For the sake of brevity and impartiality, I will withhold my personal opinions on which implant I feel is best.
Unless you are starting out with quite a bit of natural breast tissue, you can expect that saline breast implants will have a firmer feel than your current breasts.
Since saline breast implants are more rounded than silicone, they will create more fullness to the upper portion of the breast (often called "upper pole fullness) which naturally-large breasts don't have.
Since saline breast implants are not pre-filled before your procedure, the incision site can be much smaller than that of a silicone implant—typically 1" to 1.5".
As you probably guessed, saline implants are filled with sterile saline (sodium chloride)—salt water. In fact, this is the same fluid that is used for IV's, rinsing contact lenses, or cleaning a new piercing. It is completely non-toxic to your body; so in the event of a rupture, the fluid would simply be absorbed.
Currently, saline breast implants come in higher fill volumes than silicone implants. And since they aren't pre-filled, they also have the ability to be "overfilled" by your plastic surgeon to decrease the chances of "rippling" (a potential complication we will cover in a moment). It also allows your surgeon to make any on-the-fly adjustments to your fill volume once the implant is in place during plastic surgery.
In most saline ruptures, the implant will deflate fairly quickly and could be fully deflated in a matter of just a few minutes.
Most Common Complication
The most common complication with saline breast implants in called rippling.
Saline breast implants have a fill port on them that is used to inject the saline once a plastic surgeon places the implant into the breast pocket. As with anything, more variables lead to less integrity.
Saline breast implants typically run about $1,000 less than their silicone counterparts.