IMO, the Kessil will be a better light "functionally". The Radion will be better in overall spectrum and ability to fine tune and all the other nice bells and whistles it comes with, meaning you can get much warmer colors out of the Radion but that doesn't mean it is beneficial for coral growth, mainly our eyes/preference. The Kessil A350 Tuna Blue is manual dimming with knobs but has a better spectral output for overall coral growth (more focused on the lower blue spectrum).
Personally I would go with the kessil A350 over a Radion, at least 2 of them obviously on a 120G. My only real dislike with the Kessil is the manual dimming but in terms of functionality/growth that doesn't matter. The Radion is a nice light, but not $700 nice for the output you get. I like the Maxspect lights over the Radion though due to the ability to easily customize/swap out LEDs and optics and mounting options. Spread is dependent on how high you mount them. I believe both the Radion and Kessil A350 with 70degree optics would usually be mounted about 12-14" above the water to get good spread on 1/2 a 120G but I think you will get much more punch out of the Kessil in terms of penetration/higher PAR deeper in the tank due to it having more blue output.
