I haven't had Marmite in quite a while. But if memory serves, the savoriness (smell and flavor) was really more intense -- it's hard to describe, almost like burnt steak ground up into a paste. Vegemite always seemed to be more palatable to me.

I guess the best way to compare the two is Guinness extra stout vs. regular Guinness.

Unless you really like it, you probably won't use much of either -- it can go for a very long time. I personally like to use it like a seasoning to add savoriness to my cooking.