T12/8/5 Differences?

[Vbkid]

So I'm expanding my grow rack to a new shelf, which means its time to invest in some additional lighting. Thinking I just needed some additional T8s, I headed to the Depot only to quickly realize I have T12s currently. They have 2-ft 2-bulb fixtures for T12s/8s/ and 5s...and I have no idea which are best. Will the bulbs be a bigger factor than the fixture itself?
Waiting to make a purchase until later, when I am more fully educated on the different numbers meanings.
Thanks,
Kyle

 

[twigs]

T5's are the best and by far the most expensive. I personally use T8's but I'm looking to upgrade. I don't have any experience with T12's but I think they are less efficient than T8's.