I was just going to get myself a small jar because I've been curious about it for years (big umami fan). Now I see it's available in a tube too? Oh the treachery! Is there a difference?
I've used both and never noticed a taste difference and the ingredient and nutritional information is identical. The main reason the jars tend to be more popular in Australia where it's consumed more often is largely a matter of cost. Just checking at a popular supermarket (Coles) and at the moment a 145g tube is $AUD4.11 ($2.83 per 100g) whereas a 600g jar is $7.00 ($1.17 per 100g).
The tubes are convenient when travelling or camping though and less messy as MandoMando mentioned. Although one disadvantage is that you won't be able to make a vegemite sandwich in the traditional way:
You usually want a fairly thin amount spread and the tube might be better for that as a butter knife might pick up crumbs and return it to the jar.
The tube might also be better for the types that pack vegemite tube a few bread slices and go. The glass jar and knife is not as friendly with personal transport :)
I haven't seen the label on the tube but unless you're hooked on the stuff and having it daily, the jar will be fine if not better.