with T5 i don't think you need more than 2watt/gallon (this would be high light). But as I said other factors are more important to consider.
If it was me I would buy the 2 bulb T5 light with decent reflectors for this tank. But you still need to consider the placement of the light (height above tank). You will end up with medium to high light so don't forget that you will need pressurized CO2 and a regular dosing regime to keep up with demand of the plants (higher the light the more CO2 and ferts required by plants).
Trust me take a look at Hoppy's thread there are some easy to understand charts that will help determine your levels of light.
Edit: get the light that fits your tank. Don't overhang them.