The Radion G2 is slighly more powerful than the Hydra, however the Hydra has more spectrum density plus UV (the G2 does not have UV, though the Radion Pro does have UV at several hundred $ more than the G2). By many accounts 2 Hydras output similar to 1 Radion Pro, though the Hydras will cover slightly more area (and still save you a few $).
Like anything personal preference to manufacturer will prevail and you will find different opinions from different people
I did much research before going with the Hydras, and I still have 8 more on different tanks which I will be keeping and my Corals seem to love!