Griptac is a massive improvement over the stock rubber. Leather will look great too, but you'll lose some of the grip you're used to.
Morgan (Camera Leather) does support the Bessa: http://www.cameraleather.com/bessa/
And here's a look at Griptac: http://www.cameraleather.com/colors/griptac.htm
Beware, though - Morgan has been known to take his time with some orders. I can't say that I've experienced that personally, but I've heard people say they've waited months.