The two models have the same light strength (600 nits). - I have both models. You won't notice any difference when reading.
The big difference(s) in real life between the two models are that the 11" has the LIDAR function and cameras to match, so you can make 3D scans of rooms and objects. - And that the newer 11" has more "power". But with my very different 150 apps this extra power does not really make any difference. None of the apps are games.
The 10.5" can be used for many years to come for things like reading. At present it can do all the same things as the 11" except the 3D scanning. Which will continue for a long time as developers want their apps to be sold on more than just the newest M1 iPad pros.