Lighting depends on a wide range of factors including:

How close can you place your light to your subject? Light becomes exponentially less bright (1/distance^2) the further the source is from the subject.

What types of modifiers are you going to be using? Different modifiers will diffuse or scatter different amounts of light. Some modifiers may result in a 50% or more reduction of light on your subject.

How focused will the light be? If you are using a modifier to "spread" your light across a wider area, you will have less brightness than a focused light.

How bright will the background light be? If you are planning on shooting outdoors, the brightness of light can vary quite a bit. For example a sunny day with snow on the ground can produce extremely bright light, while shooting at dusk on an overcast day would be much less bright. If you are thinking about using only one light, then you will need to consider if you will be able to light your subject evenly, or if you can use ambient light to even out your monolight.

With regard to using the light indoors - you can always adjust the power level, bounce the light, or place a modifier between the light and subject to make it less bright. I personally don't see a downside to using a brighter light inside, as a bright light can always be made 'less bright' (although it may produce unnecessary heat). You can never make a less powerful light more powerful...

Personally, I would imagine that a 600WS monolight will provide you with sufficient power to fill in ambient outdoor light in most situations, even with a modifier that reduces the light by 1/2 or 1 stop while still being useful indoors.