I would pick the RB250/incognito package. The wiring on the Rega arms really can be improved, and the 250 is actually a better sounding arm than the 300. When fully modded, a modded 250 will outperform a modded 300. The tapered tube on the 300 is nice, but the spring loaded VTF device is a disadvantage. Overall the 250 wins. But you need to put an OL end stub on the 250, because the original plastic end stub is not good. Also the Heavyweight counterweight is a very good upgrade for any Rega arm.
