I switched from high quality leather leashes to biothane and will never look back. It's supple, can "pop" a correction as good as leather and doesn't get funky like leather when wet or muddy. I haven't experienced any slipperiness.I have a 6ft with a frog connector, a 30ft and a 2ft lead; all from mydogequipment.com. They are high quality, custom made in the USA, NOT CHINA!
I use my 30 foot line daily. It feels great, but I have added 3 knots along the length, as it's slippery when wet. The drawback of knots is, after a while, (4 years with my first long line), it broke in 2 where a knot was. I move my knots every week to prevent a breakage. Also, you want the leash stitched, not riveted. Riveting compromises the integrity of the leash. There is a non-slip biothane version available, the biothane is textured. I prefer the feel of the smooth version.