A single decanter or a set with glasses
The first question is nearly always: on its own or as a set. If you already have tumblers you are happy with, take a single decanter. If you are furnishing a bar cabinet or looking for a gift, a set makes more sense, because there the decanter and glasses share the same cut. The Opera whiskey set by RCR has seven pieces, and Luigi Bormioli supplies a Mixology decanter of 75 cl with four tumblers of 38 cl.
Choose the capacity to match your bottle. A standard bottle is 70 cl, so a 75 cl decanter takes it exactly, with a finger's width of air above. The 50 cl models are for part of a bottle or for a cramped sideboard. Finally, look at the stopper: cut glass with a snug fit seals better than a loose cork.