If you want to keep the genetics alive (or just want a guaranteed female plant for your next grow), it is preferable to take clones before flowering. That way you can have your next plant all vegged and ready to go when your current one finishes.
You can reveg after harvest, but it is a slow process, and there is some debate as to whether revegged plants can have weakened genetics. Clones are more of a guarantee, plus you save some time between grows.
The downside of clones is the need for a second grow space, on a veg lighting schedule, although it doesn't necessarily have to be anything more than a small box with some CFLs.