What corals do you plan on keeping?
If softies and zoos, you may be able to get away with two. If SPS, I'd recommend a minimum of 3.
But either lights are amazing. Depends completely what your budget is and what features you like more. Which is also kind of hard since they both are similar in features.
However, the Hydra 26HD does include the wifi inside the fixture, so you can easily control them from your phone. Where as the Radion XR15, you will need to use the Reef link (Extra $$) to be able to control the light from your phone