Quick list of relative merits between saline and silicone:

• If the implant is cohesive silicone gel and the shell ruptures, there will be no leakage or deflation; this is not true of conventional silicone gel.
• Silicone implants are sealed at the factory, and therefore are not an option for the TUBA (belly button) procedure. It's unclear whether they're an option for the transaxillary (armpit) procedure; depending on size, this could be "difficult" to "not an option".
• Saline implants can be overfilled. If you prefer the "fake look", this should be your choice.
• If a saline implant shell ruptures, the deflation can be slow, or it can be quick.
• The Becker implants seem to be a compromise, by being a hybrid of saline and silicone (separate chambers, where the saline bladder is inside the silicone bladder). Some swear this is the best option out there. However, some sources have given their (professional) feedback of this particular implant which has been less than stellar (ie, the fill port valve has been known to leak). Note that this info might be outdated; that said, caveat emptor. Check with your surgeon; if he prefers them, then chances are he's had great success with them.