T12 is pretty old technology. I am using mostly T8, because a few years ago when I set up most of my tanks, T5 was still very expensive. T8 is supposed to be a lot more energy efficient than T12, but T5 only a little more energy efficient than T8. When I last checked at the hardware store, T5 seemed to have come down in price a lot - and it is the best fluorescent lighting available. If you get a good price on a T5 fixture, I'd say go for it. You will enjoy it!
My dad recently upgraded from T12 to T5, and he was amazed by the improvement. He says his plants have been growing like crazy since he changed over, but I think he also added an extra tube. Probably not a fair comparison, but T5 is just so much more compact that it allowed him to fit three tubes in the space of two T12 tubes.
One more thing to think about, LED lighting is the new kid on the block, and in a few years everybody will want that. In fact, I think the competition with LED is what has brought T5 prices down. But T5 is probably by far the best value for money you can get right now!
