it depends on your growing style. i prefer T12/T8 to T5 because of the heat factor. T5 lights throw off a good bit of heat (for a fluorescent light) so you have to keep the plants at least 4-6 inches away. with T12/T8 lights you can purposely slam them into the bulbs with no issues what so ever. this means that absolutely no light is lost.
i am currently using a 4' 6 bulb T5 light as my main light and have one 4' 2 bulb T8 light in each corner as supplementary lighting. i also have another cabinet with one 4' 2 bulb T12 light that i am growing some autoflowers under.
in your case, i would simply buy a few cfl lights to supplement what you already have. if you were going to growing under them for a long time, i would possibly upgrade.