There are better choices than the T12 or T5 tubular fluorescents you recommend, specifically T8 types, which you did not mention.
Measuring the illuminance 3" below two-lamp fixtures, T12 gave 12000 lux, T8 gave 16,000 lux. T8's also have a 50% longer rated life, use 80% as much energy, and cost only slightly more. Furthermore the 2005 Energy Policy Act has established minimum efficiency ratings which eliminate T12 lamps from new installations, which means that they will be scarcer and more expensive in the future.
There are two kinds of T5 bulbs. The original ones have about the same output as T8s but cost more. The newer "High Output" T5's have 65% more output, but cost more and run hotter so you don't want them as close to your plants. T5 fixtures also cost 4 times the price of T8 fixtures. It might be possible to use one T5HO fixture at a greater distance to replace the two T12 fixtures on one of your shelves, but it would mean rebuilding my light rack and still had a higher cost, so I did not consider using T5's.
You didn't mention LED fixtures. There are some available now which are slightly more expensive than T8 (if you include the fact that the bulbs are permanently included and that they have a potentially longer life), are available in the "cool white" color, and produce 16000 lux at 3" distance.
So the moral is, T8's are probably the best way to replace your old T12 fixtures. I have oly tried LED's for a short while, but they may also be competitive to T8's, especially if their price comes down a bit.
Note: I use two 2-lamp fixtures per shelf spaced 10" apart to cover 4 selling trays. This gives a more uniform coverage and costs less than one 4-lamp fixture. All my lamps use "Cool White" 4100°K lamps, which have a little less blue light than the "Natural" 5000°K lamps you recommend, or "Daylight" 6500°K lamps which have more blue light than "Natural". Both are less expensive than "Natural". I notice no difference between using 2 of these or substituting 1 soft white (3500°K) lamp in the same fixture. By the way, the expensive "full-spectrum" grow lights add more light on the red end which is useful for plants in their budding or fruiting stages, but not for growing seedlings, which botanists say need more light on the blue end of the spectrum.
