Because overdriving them creates more PAR. A brighter bulb. Why spend another $120? Because Icecap is worth it.

The bigger better question I cannot believe has not been asked, see it's funny to watch people toss out answers to questions they do not know, what size tank? Dimensions? What are the plans of the tank, IE: CO2, what type of plant, fish? All of these could help make a better decision. Examples, some plants like Anubias can grow well in lower light situations, those of T12-5. Other plants, generally with the color red, must be grown under halide, 10K or less. Things like the depth of the tank can quickly influence what type of lighting is necessary.

With plants most any tank 16" or less in depth I'd choose T5 (but remember it takes lot of them, not just 2). If the tank was a lot deeper than that and had plants needing a lot of light, halide would be my only choice.

You do have one other option with a freshwater planted tank that you don't see mentioned with reefers, sodium vapor or mercury bulbs. They are much cheaper than halide but do not offer any color spectrum. Basically a dull yellow is all you'll get.

I'd give the very nice folks at Texas Hydroponics a call as this is what they are in business for.