Nghiep,
I've compared the 901 and RSP, in two systems and on several arms (Graham 2.2, Basis Vector, Schroeder Ref, modded OL Silver, TriPlanar VII).
In general I liked the review Fretless posted, and I also agree with him that the 901 lacks the midrange "magic" of the RSP. For me that is a good thing, since I find the the RSP's midrange "magic" intolerable. OTOH, if that's the kind of thing you like, you'd like it very much!
The 901 has stronger bass, crisper highs and greater macrodynamics. On some arms it gets to the point of being edgy, which the RSP never does. OTOH, the 901 on a Schroeder Ref sings like a Koetsu, but without that rounding and gentling of transients that Koetsu lovers love. Until I got a ZYX that was perhaps the nicest cartridge/arm combo I'd heard.
How these two cartridges sound is very arm-dependent, especially the 901. Which one you'd prefer is very you-dependent.
Doug