Depends on quality and objective size. Good quality, 80mm or so objective, up to 60 power. Good quality, 60mm objective, 45 power is better. The big difference is low light conditions. Also what you want out of your spotter. 80mm's are bulkier and heavier, but have a wide field of view and gather light well, along with being able to handle more magnification. 60mm's are more packable and lighter, yet still do a good job. I personaly like the 80mm class. I have a 20-60x80mm and I wouldn't trade it for any other spotter on the market.
Something to understand is, as magnification increases, light gathered decreases, thus making it darker. Quality of glass is just as important, probably more important, but that is a different subject.
No matter what the quality and objective size, less magnification is brighter and clearer.