I've put a fair bit of thought into light... and I can tell you (as far as my logic tells me) that light spectrum is not the only important aspect to growing weed, but also light intensity. The more intense the light the more potent your plant will be.

LED's are very similar to fluoro's in that they put out very little intensity. You might get it to grow, might even one day pull a decent yield, but it'll never be as potent as the same plant grown under a HID.