The biggest thing to keep in mind regarding the choice between T-8 and T-12 lamps is that the T-12s are being phased out of production within the next couple of years.The government's energy saving legislation will basically outlaw the production of T-12s after a certain date-(2010?).
IIRC,the lumens of the two types of lamps are pretty much equal for comparable length fixtures.For the most bang for the buck in flourescents,the current way to go is with
4' T-8s. If you have an open (unfinished)cieling or are hanging the fixtures down from the cieling,get industrial fixtures with reflectors.If you have a white finished cieling,use 2 lamp strip lights or even flange mounted troffer (recessed) fixtures.
There are several members here who have access to layout programs -maybe one of them can help you with placement of the fixtures. Be sure to go bright enough with the general lighting so you can work anywhere in the garage,then add additional lighting in close work areas on a separate switch.
George