Always been a Marmite man. Tried Vegemite for the first time the other day so I thought I'd share my opinion with anyone unfamiliar with it.
On dipping my knife in it became obvious that Vegemite is considerably more viscous than Marmite, with a consistency and spreadability akin to chocolate spread. Once smeared onto buttered toast - my testing platform - it was ready for tasting, and to my surprise it had a less saline flavour than Marmite. This was reflected in the ingredients list: 3.5g per 100g versus 3.9g. Overall it has a similar but more subtle flavour, which is no bad thing given that Marmite can be a little over-powering for some (hence the 'you love or you hate it' branding).
In conclusion I think I will probably stick with Marmite as I'm already used to its 'salt kick'. It's also slightly easier to spread. Vegemite, however, is a good substitute.