The natural bright sun is always a better option, as it provides the complete spectrum of light that your plants need. However, it is difficult to turn the sun on and off when you need it to, LOL. Secrecy issues are also a concern when growing outdoors.
As long as the days are at least 16 hours of direct light, the plants will not get triggered into flowering, or shouldn't. Using pure sunlight means only the decreasing daylight hours will trigger flowering. If you have the time to wait till fall, go for it. If you're trying to do this quickly, and need to be able to control your lights, then obviously using the sun isn't viable as the only source of light, but direct sunlight as a suppliment would be great.