This question comes up very frequently and can be a difficult decision for both surgeons and for patients. Some surgeons are very flexible in the implant offering and some strongly believe in one implant over another. Whenever this question comes up the opinions will diverge as there is no consensus.
The saline breast implant is made of a silicone rubber elastomer which makes up the shell or outer covering. The implant is placed empty and then filled therefore they can be placed through a very small incision around the nipple, under the arm, or in the fold under the breast. The implant will wear over time and the saline fill will leak, perhaps over ten to fifteen years after which the implant is easily replaced. In a submuscular position the saline implant has a very low capsule contracture rate and produces a soft and natural feeling augmentation. The relative softness as compared to a silicone implant is not all that different in the breast. The relative drawback to the saline implant is the tendency to have a fold or ripple along the lower edge or side of the breast which is not as commonly seen with a silicone gel implant. The advantages remain the small incision, lower cost, no need for special testing to detect leaks, and the implant can be worn until it wears out with little risk, and one more; saline has a lower capsule contracture rate (firmness).
The silicone gel implant is also made of a silicone rubber elastomer with a coating to lessen the gel leakage. The implant is filled and therefore placed with a larger incision, usually in the fold under the breast.. This implant too will wear over time and the leak will cause the implant to become tender and the capsule may begin to draw in increasing the firmness of the implant. The volume will not change like the saline implant. The implant will last as long as the saline implant, however as there is no warning of the leak, the FDA recommends an MRI study after three years to check the implant and regularly thereafter. This study is not a mammogram and probably not covered by most insurance. The implants are soft and look well and have less ripple than the saline implant. The firmness or capsule contracture rate is higher than with the saline implant, and unless removed before a leak occurs all will become firm over time. If the implant leaks, it is more difficult to replace, often requiring removal of the capsule around the implant as well.
There are some patients who will do better with a gel implant, for example if the implant is very large, over say 550cc, and there are other considerations as well. My bias is that if a patient will look well with a saline implant it is a safe place to start. Both are very good and it will take you time to consider your comfort level to feel out which is best for you. I hope you can find a surgeon who can spend the time to consider both options.
Best of luck,