You could do it with either.
One advantage of using the SB900 is that is has a feature called Auto FP High Speed Sync. It allows you to use any shutter speed on your camera, which is a big advantage when you want to overpower the sun. The problem with this though, is that it really limits your working range.
With a studio light, you will be limited to the max sync speed of the camera, so you would need to close down the aperture to keep the sunlight in check...and as you close down the aperture, you need more and more light power. The SB900 is pretty powerful as far as Speedlights go, so depending on the situation, you may be be able to avoid Auto FP and stick to the max sync speed, but a good studio light (with an appropriate modifier) will give you more light to help overpower the sun.