Lumens describe the light emitted by a lamp, while lux describes illumination on a surface. The numbers are not interchangeable: lux also depends on distance and how the light is distributed.
Look at beam images alongside the specifications. Consider the illuminated width, the road immediately ahead and the beam boundary. One bright central spot does not describe the whole illuminated area.
Compare runtime in the same operating mode. For example, CatEye lists the AMPP900 at two hours with 900 lumens, three hours with 400 lumens and five hours with 200 lumens. Its 35-hour figure applies to flashing, not continuous 900-lumen output.
Before buying, check mount compatibility and charging convenience too. Start with your ride duration and road lighting, then compare the beam and runtime of an appropriate mode. This makes the specification a connected set of information rather than a single headline number.