I'd go for ones with a nylon core as they don't stretch and are strong, I've even used the synthetic ones (wintec and thorowgood) on a leather saddle before for that very reason, but there are some nice leather covered ones now.
I've just got the GFS ones which are fab. They're non stretch nylon core covered ones and the holes are closer together than normal so you can 'fine tune' your position. I can't find the exact ones on the internet but I got them from "The Saddle Consultant" for about £30. I hasd the wintec ones for ages and never really likes tham. OK they don't stretch but are very plasticy and don't roll around the bars easily.