With memory gel, the doctor uses what is essentially a silicone gel. It holds together well and even mimics the give of real breast tissue when pressed. Doctors can provide a smooth or a textured surface to these breasts, which can feel somewhat realistic.
When using memory gel surgically, the doctor must create a large incision. He will have various choices for where these incisions will be made. When using the memory gel, he is limited, however, to a set volume of how much gel can be placed inside the silicone sac, so there is a limit to the breast enhancement.
Saline is a bit different than memory gel regarding its feel. It has a more firm feel than the natural give of memory gel. It contains a solution of saltwater, which mimics the fluid in humans --- so while firmer, it also imitates the feel of the human body as well. Doctors can create smooth or textured surfaces to these breasts.
Doctors make a small incision when using the saline method for breast-implant procedures. He still has options for where he will make his incision, but the main benefit of this option is that he has a flexible fill volume for the sac. This allows him to adjust the breast to different sizes during the operation.