I'm using two t-12 ballasts right now, and I've used the t-8.

Used cool white bulbs only in the t-8 with disappointing results. I'm using one full spectrum (but heavy on the cool white) daytime bulb in each t-12 ballast, and one full spectrum (heavier on the warm red) sunlight bulb. The t-12's with two types of full spectrum bulbs in each ballast are blowing the t-8 away. Will definitely use the t-12 again next time. Growth is fast and healthy.