IMO, Dre is better but that is just my opinion as I prefer his beats.
I do agree that RZA is dope, it's just his beats, IMO, are a little more difficult to appreciate. Not to take away from him as, after all, much of what has went on to be recognised as great was once perceived as being a bit weird.
That said, I don't think Dre could produce such a gritty sound as RZA did for the Wu-Tang debut. Yeah, Dre did drop some harsh beats for NWA but there asn't the same grime that RZA created.
Still, while I've heard less of RZA's work than I have that of Dre, I'm not sure if RZA can produce the smoothness that Dre has consistently dropped.
Anyways, basically I think their styles are contrasting & I prefer Dre.