I have a RB67 Pro (no S or SD) and have zero complaints. Well maybe the weight, but I too can live with it. I think the main difference between the two is the RZ has the built in meter. Which is likely why it's more money.
I see a bit more stuff for the RZ than the RB (but not by much) on e-pay. Never used an RZ so I can't really do a fair comparison.
I got the RB simply because I had the money at the sametime it was offered up here by a fellow APUGer.