I have the BP6 myself and it's an outstanding pre-amp but let me share the results of my research on how it compares to the BP26:
The BP26 theoretically and by specifications has a lower noise floor and lower distortion; however, in the real world, this alone may not be detectible. It would depend on your hearing sensitivity, your listening rooms control over ambient noise, the transparency of your system - your cabling, your speakers etc.
A key difference between the BP6 and the BP26, as well as former Bryston preamps i.e. BP25, BP20, is that the BP6 does NOT have balanced (XLR) in/out. A balanced connection between your preamp and power amp will give you a higher output voltage and a resulting lower noise floor. Whether this increase in noise floor will also depend on the other factors previously mentioned; however, some on this forum have already stated that they perceive more dimensionality and a lower noise floor - darker quiet sections with XLR / balanced vs. single-ended RCA connections. This also may be dependent on other parts of your system.
All in all, you sould seek a comparisoin before buying. If you just can't afford the BP26 or can't budget for it then go with the BP6 - it's a great preamp in it's own regard.