Speaking as an Irishman, brought up in England and now living in Australia I would urge caution when comparing vegemite with marmite :rofl:
Aussies are convinced that marmite is inferior to vegemite and the Poms consider vegemite to be inferior to marmite and possibly even the spawn of the devil (true lovers can get quite excitable about the merits of the various products - personally I like them both). One of my fellow ex-pat friends stocks up on Marmite every time he returns to the UK (no idea why; it's easy enough to find Marmite in the major Aussie supermarkets!).
One indication of how important Vegemite is to Aussies is that one of the final things you encounter in airport duty free are toothpaste sized tubes of Vegemite by the cashier - handy for when traveling to uncivilised parts of the world where it can be difficult to find the stuff (i.e. anywhere that isn't Australia

) No true Aussie is putting PB and J on their morning toast...
Newbies to vegemite/marmite might struggle to tell the difference but marmite is slightly saltier than vegemite and both should be used sparingly (at first; hard core lovers of these spreads can eat it by the spoonful).
As others have mentioned; both are love/hate products but the link below might provide enlightenment to anyone who is unfamiliar with these spreads; it is from the English edition of the Guardian newspaper and rather eulogises Marmite but I've seen similar articles where the roles of Marmite and Vegemite have been pretty much reversed!
article in the Guardian re Vegemite and Marmite