People ask me all the time — yellow or brown? And I tell them the same thing every time: it depends on the sandwich.
Yellow mustard is bright, tangy, and cuts through rich meats like bologna and ham. Brown mustard — the spicy kind — is what you want on pastrami, corned beef, anything with some heft. And then there's the whole-grain stuff, which I'll admit has grown on me for turkey clubs.
The point is, mustard isn't an afterthought. It's the bridge between your bread and your meat. Treat it with respect.