I would go with the 4x14 sinc it's got two switches. At 28 watts (two bulbs on), you have enough light for low light plants. While at 56watts, all on, you are just under 3 watts per gallon, the recommended general lighting for medium to high light plants. Really you have more options with that light, and with four bulbs you can mix your spectrum bulbs to get a cooler more customized lighting for your tank.
Steward of the Tank