I have both the Haynes and the Bentley book and have found it useful to refer to both of them on more than one occasion.
Often, one will have a diagram or photo, where the other does not, that shows exactly what I am looking for - for example, I am currently stripping down the pedal assembly because I want to replace the roller pedal with a flat accelerator pedal. The Bentley, gives a description of the process, but no diagram. Whereas the Haynes does not have as good a description, but does include an exploded diagram.
However, if you want to strip and rebuild the rear axles on an IRS then the Bentley has much more useful information, including an exploded diagram of the axle assembly.
It's a personal decision; based on how comfortable you are with the work, and how much you would use the manuals.
I did not intend to buy both manuals. But now that I have both, I am glad I have more than one reference.
If I had to mke a decision I would go with the Bentley, but that's my own choice.
Jim.