If you already have the Gen 5 it just a little more blues and the spread is a little more but not much. I have the Gen 5 and seen the Gen 6 in person and thought it was not worth upgrading to the Gen 6.
If you have the Gen 4 than I would consider upgrading to Gen 6. The Gen 4 still are good lights IMO. I had the Gen 3 pro and it was night and day when I upgraded to the Gen 5. Lights are so expensive nowadays, but if you have the $$$ than I would go for it.
The quality of the LED light from G4 pros are better than the newer G5 blue/pro if you're a type of person that uses a lot of the blue spectrums. If not, the newer generation might be better. I'm not entirely sure. I get the spread of light is important but the quality of the LED light is more important IMO. The newer G5 blue/pro has more LEDs for better spread but they're smaller, thus, not as powerful as G4 pro. Another thing I don't like about the new gens is that they have this aqua blue/green color in the royal blue spectrum whereas the G4 pro has this crazy deep purple royal blue color. Big difference. Half of the guys I talk to says the same thing about the blue colors. They like the G4 pros better. The others seems to like the newer generation.
My take, I wouldn't upgrade. G4 pro is one of the best LED light in reefing today.