I use nutritional yeast or nooch as it is sometimes called, a lot. It's great for adding the b vitamins into vegan food. It's a nutty flavor, some say cheesy though I don't really get that taste. Nutritional yeast is something I add to a lot off my vegan 'cheese' sauces, last night's vegan stuffed potato soup, and even some of my vegan cheeses. I use it by the tablespoon or even by the cup full (US measurement system). It's great for that extra boost for flavour. I'm from the UK (until 5 years ago) years and I'm familiar with the brand you linked to but tbh, it's not my favourite one, and the added B12 was my least favourite of the 3 versions that are made by Marigold Health Foods. Nutritional yeast contains all 9 essential amino acids, is high in protein, fibre and obviously vitamins.
Marmite or vegemite is a totally different concept and exceptionally salty. It's a thick almost black or dark brown sticky paste. It's always a case of use with caution, add a little at a time (¼ tsp to 4-6 servings of soup) or spread it as thinly as possible on toast or sandwiches (its great with cheese). The manufacturing process is totally different. Marmite is glutamic acid-rich yeast extract made from the beer brewing industry, it's a by-product, so left overs. It then has numerous vitamins and minerals added, so many in fact that some countries (e.g. Denmark) have banned it.
Just look at the nutritional labels of both products and compare them. The difference is considerable. Marmite contains no protein or fibre. It's also very high in salt hence the low salt versions that are around.
I don't really use marmite/vegemite but I get through loads of nutritional yeast
