I also avoid leather products and found the Brooks Cambium C17 to be very comfortable. Although as you can see, not everyone agrees on that. However, when I mulling it over, I decided to ask one of the guys at my local bike shop, and he said that pretty much everyone in the shop ended up putting a Cambium cutout version on their bike after trying it.

But look at the other thread that was just started about this saddle. Brooks has just released another version that is listed an all-weather and more durable. The materials are slightly different, and I don't know how well it's been tested yet, so I don't know how it will compare to the original Cambium, which I thought was also "all-weather."