Obviously I am a bit biased towards synthetic, but I try to give out as much non biased information and allow customers to make informed decisions instead of trying to push a product. With that being said synthetic lines can last a very long time. My Superline XD is almost a decade old and is still in working order, I have seen a few customers send theirs back after 7 or 8 years to have a new thimble or hook put on and the rope is still in good shape.
Basically when it comes to synthetic you have 3 choices for Dyneema and a few others. You have your open weave 12 strand ropes (MP Basic, Amsteel Blue, Viking Trail Line, Spydura, and so on), then you Superline (which has been heated and stretched to give increased strength and abrasion resistance), and finally Superline XD (this has a core of Superline with an outer braided cover along the entire rope). The braided cover on Superline XD helps with abrasion resistance, but more importantly it keeps the dirt and grit from tearing up the inner load bearing core. We have seen multiple failures due to age on our basic series that are caused because the rope is dirty on the inside and the grit has been working like sand paper on the rope.