Personally, I like them both. They're pretty much equal IMHO. What one has the other doesn't.
The main thing that I liked about RX was the last 10 episodes which is the last time all of Ishinomori's Riders had a television teamup. I like RX because he was one of the only Riders to beat the Dai Shuryo without any other Riders helping him in the final battle against Shuryo himself. I also like the fact that RX was the first Rider with extra modes.
As for Black, I like his simple and varietyless but powerful ethic in choice of weapons/attacks. All he needs to finish a kaiju are just a Rider Punch and Kick. Hardly any Riders before or since have relied solely just on those basic attacks. And Black just has one of each; he lacks variety. But what he lacks in variety he makes up for in sheer power.
Story-wise they're equal, but RX is special to me because it closes the book on the Showa/Ishinomori Era of Kamen Rider.