It's all about the feel. Frankly they both look exactly the same. The more of your own tissue you have, the less difference there is. For instance, if you are a thin individual and you are wanting to go from a "B" to a "DD" then you may probably feel that the silicone implant feels more natural. If you have a fair amount of your own natural breast tissue and want to go up one or two sizes the difference will be much less noticeable.
The costs of saline implants run about $1000 dollars less than silicone. They both have 10 year 'part and labor' warranties (like a KIA) and lifetime implant replacement policies/ The silicone implants are lighter for the same volume but only minimally.
If you are worried about the health concerns associated with silicone implants.. forget about it.All this was resolved in 1998 when no (ZERO!) correlation was shown between silicone and all the diseases that were implicated (from Lupus to Loss of Libido). So get on the net and get informed, there is a right implant for you just waiting for its chance to get you the shape you desire.