The most important difference between the two temptations is the difference between life and death. If Odysseus' sailors eat the lotus, they forget about home, but they live. If, on the other hand, they succumb to the alluring songs of the Sirens, the sailors will die because their ship will crash on the rocks where the Sirens perch to sing. (Some versions of stories about the Sirens indicate that men are so enchanted by their songs that the men jump into the sea.)
A second difference lies in the sense to which each temptation appeals. The "honeyed" lotus appeals to the sense of taste while the Sirens' beautiful song is something the men would hear.
In both cases, evasive action must be taken to save the men. Odysseus has to go ashore to rescue the men who have already eaten the lotus. To avoid hearing the Sirens' song, he plugs his sailors' ears with wax. Because Odysseus wants to hear the lovely song, however, he has the men tie him to the ship's mast so he cannot steer the ship to sure disaster when he inevitably gives in to temptation.