If you want to relax more, Lake Naivasha is nice. I've stayed at the Lake Navasha Sopa a few times and it's nice, bur fairly large so the dining area can be a bit noisy. Often you will see animals such as water bucks, giraffe and hippos near your room. You can take a boat ride on Lake Naivasha for an additional cost and if you do that, I'd recommend doing it in the morning as the water is calmer than in the afternoon.
But if given the choice between the two locations, I'd take Nakuru any day. More wildlife to see in the park - rhinos, lovely giraffes and I've often seen leopard there. I've not stayed at the Sopa in Nakuru, only the Sarova Lion Hill which is nice.
Laurena