Lighting a newly planted dirt 15 gallon..?

[LyzzaRyzz]

Just up graded my ten gallon to a 15 gallon, and my lighting situation doesnt work anymore!
Im running it low tech, but my plant variety is quite vast. Right now, and before on the ten gallon, i was running twp 14 watt energy saving spiral bulbs, and that left a yellow tint to my tank. I was thinking of upgrading the tank anyways, so ive been looking for fifteen gallons, and suddenly got everything i needed for the upgrade, and went for it! Whats left is a few plants and the light!
On my other tank, i have a single 48 inch t8 6700k, though, i tried to find 6500k, and couldent.
Should i go t8 or t5 with the 15 gallon? Before i already had a fixture, now im starting fresh.

Anyone have an idea? Its the 24x12x12