A classic loafer is a slip-on with no laces and a low heel. What separates one from the next is the small detail across the front, over the foot's instep. Three of them have lasted for decades: the penny (a leather strap with a slit or diamond cut-out), the tassel (two small leather tassels on a lace) and the horsebit (a metal bar in the shape of a horse's snaffle bit). They share a silhouette, but they say different things.
The reason is visual weight. A detail is a point of interest at the front of the shoe, and the eye goes straight to it. If it is flat and leather-on-leather, the shoe stays calm. If it moves, it adds a playful note. If it is metal, it catches light and reads as dressier. Which one suits you depends on what the rest of the outfit is already doing.
What are you wearing it with?
Two people can wear the same black loafer and look completely different, and it is almost always the front. Take the detail away and you have a shape: a low profile, a rounded toe, a flat sole. Add a strap and the shape gets a graphic line. Add tassels and it gets a bit of movement. Add metal and it gets a highlight.
That highlight matters because the foot is far from the face. Anything at floor level is seen in glances, so a small contrast carries a lot of the message. A bright metal bar on a dark shoe is the loudest of the three; a strap in the same leather is the quietest.
The detail is the punctuation mark of the outfit. A strap is a comma, tassels are a smile, metal is a full stop.
Start with what you already own. If your wardrobe is mostly denim, plain knits and simple coats, a penny loafer is the easy fit: it works with all of it and never needs special treatment. If your clothes are already quiet and you want the shoe to add a little personality, tassels do that without shouting. If you wear tailoring, or a belt with a visible buckle, a horsebit picks up the metal and ties the two together.
Match the metal if you can. A gold-tone bit goes well with a gold-tone watch or buckle; mixed metals can still work, but then keep it to one thing at a time. And note that the same logic applies to trousers: a hem that just touches the top of the shoe hides the strap, so a slightly shorter hem shows off whichever detail you picked.
It also helps to think about colour. Black is the formal default, dark brown is warmer and softer, and cream or tan feels lighter and more relaxed. A tassel in cream looks different from a tassel in black, so use the colour to dial the same detail up or down.
Around €20–60 buys a loafer in the right shape. Around €80–120 usually brings leather and a neater finish, and above that you are paying for materials and construction. Re-wearing a pair you already own, or looking second-hand, is a fine way to try a detail first.