It depends on the use, your budget, and what the people you typically dive with are using.
In dark, green water, and given the same focus on the beam, brighter is generally better. Most of the people I know in Puget Sound are using 21W lights, a few the newer 35s, and a few the 10W. In the tropics, at night, a 21W light is massive overkill. But if you are diving with a team of people using 21W lights, a 10W, especially in the daytime, can be quite hard to see, which limits its usefulness as a communication tool.
The Salvo fixed-focus 10W light is a very nice, bright light, even in murky water, and is quite affordable as can lights go. My light is a Salvo 21W variable focus, and that's the light most of my friends have, too.