I have leather harnesses and biothane harnesses. For a well broke horse, I prefer biothane or betathane because of the easy care. Betathane is much nicer feeling for the reins than biothane. Betathane feels much more like leather.
The nicest most expensive piece of tack ($1200 new) that I own is a Lutke harness, and they are made of biothane and so delicate to look at yet of course very strong. It's almost a work of art.
SO, I guess in most cases for "just for fun" driving or for show, I like the biothane / betathane harness I have over the leather and patent leather ones.
One thing to know is that biothane, betathane and nylon harness are probably not a good choice for training or for green horses because these materials will not break in an emergency the way leather will / can and sometimes you need it to have some give or break-ability.