It really depends on your budget, dude. I've read that vegging plants benefit more from 6400K, and flowering plants benefit from 2700K.
If you're doing a 12/12 from seed, and you have some spare money, I would still use 6400K for the first few weeks until you have confirmed the sex and they start showing early signs of flowering. I would then switch out most of the 6400K for 2700K, maybe leaving a 70/30 split of 2700K and 6400K bulbs.
It's really up to you, though. If this is just an experiment to see if you can actually produce some bud, maybe just stick to the bulbs you have and wait for the results. No sense in spending more money until you know for sure that you've got everything right for a grow (soil, pH, temperature, humidity, nutrient cycle, bug control, etc).