RA is an eastern smorgasbord. You get more instruments for your money, but I don't believe it has as many silkroad instruments as the SILK pack itself. The SILK might also have more articulations. I don't know this for sure, but generally with a more specialized library the less instruments it has the more detailed it probably is with the instruments it has. Not to say RA is not sampled well, my brother has it and it's incredible.
I personally would just wait and save for the entire damn Composer's Collection and get Symphonic Orchestra, Pianos Gold, Choir Bundle, Storm Drum 2 Bundle, Silk, Ra, and Gypsy for like freaking 70% off or whatever. That's $865 I think while they're all normally over $2000 if you bought em together. Plus it comes with a hard drive. (a nice western digital 1TB. It's not caviar black, it's blue, so it's decent but not eco-slow like the green)