l:ö (laü) is the Bija of Muladhara Chakra. —Swami Sivananda

Bija means seed, source. The bija mantra of a chakra is related to the power (the source) that fuels the chakra. Obviously, that is different from the mantra that stimulates the receptor of the energy, the chakra itself.

You can chant whichever you want. However, if the chakra is asleep, dormant, or malfunctioning, it is probably best to use the mantra of the chakra first so that it becomes awakened, balanced, and useful. If the chakra is spinning invertedly or asleep, then stimulating the energy with the bija will have no effect or a poor one.

You can relate this to a car: if the engine is not working, why would you keep putting fuel in it? Fix the engine first.