RB25 factory is rated at 225hp but in reality they are closer to 230hp from the one's I've seen.
RB26 factory is rated at 275hp but in reality are closer to 300hp.
Both have the same bore size, which mean the RB26 has a slighly longer stroke. That is where the simularities end. The RB26 comes with a much better head, better cams, stronger crank, stronger pistons, rods, much stronger block and support craddle, better oil pump, and water pump, intake plenum, etc. It is also capable of making much more reliable hp, more hp, and is over 2x as expensive.