Well congrats on the tank and welcome!!! That will be a nice tank. Either light will be good. If you like tech, you will like the prime. Some like the extra shimmer on of the kessel, but to be fair, all of these lights do that. Also, if you will have the light hanging from the ceiling or a mount, consider what appeals to your eye. Both have distinct looks and one may be more eye appealing than the other.
Now, how many :) If it was a rectangle, I could tell you for 275 L it would be two and you would have plenty of coverage up to a 340L. Once you go over that, you are getting close to needing three, again depending on the mount. For a 121 L it would definitely be 1 light.
Since you have a cube, I would start another thread asking how many Kessils 160s or AI Prime HDs people have over their 275 L cube. This will firm up how many.
How do you plan on mounting the lights? I have a 15" cabinet over my 65G (rectangle), so because it's so deep I needed two lights because some is lost on the walls of my cabinet. If you are mounting open, you can raise the light higher to get coverage, and that's where your new thread mentioned above will be helpful.
As mentioned I responded this way to avoid everyone telling you they love what they bought, I see it all the time... :) I hope this is helpful