IMHO, apply the marmite first. It's extremely viscous, so much easier to apply to unbuttered toast, and while the toast is hotter to soften it. Plus, you want the marmite underneath a layer of something else, so that it's not the first thing that makes contact with your mouth when you bite. You want the flavour to develop as you chew.

Also, marmite works really well with peanut butter instead of butter.