This is a tough question to answer. I like Little Cup because calculating EVs are tougher and it gives Pokémon whose evolutions are pretty unused (such as Lickitung and Carvahna) a chance to shine.
I mostly battled OU until I came here, and I learned the ropes to NU somewhat (at least the basic counters and checks). Right now my favorite tier is NU because I hate auto-weather.
It also has more Pokémon and therefore more variety. That means more to prepare for.