Marmite is stronger and more tangy than vegemite. There are several versions of Marmite: There's a South African Marmite, there is the standard Australian Marmite, which is made in New Zealand, and there's the classic English Marmite. In addition, there is another aussie Marmite made by Sanitarium. They all differ slightly in flavour and strength.
There is also Promite, which is a sweet and sour recipe also based on a yeast derivative. It's marketed for vegetarians, and it is our favourite of all the yeast derivative products.
BTW, you don't need to keep any of these products in the fridge.