Before anything else: count the pieces. A set can be two, three or more. The photo shows the styled look and the listing says what ships. Check which one you are reading.
On sizing: a set is sized as one unit. If you are a different size on top than on the bottom, a set will fight you on one half. That is what buying the bra separately is for.
If it is a gift: a set beats a single piece, every time. One decision instead of two, no risk of a mismatch, and it arrives looking like an occasion rather than like underwear.
If you want to wear it more than once: lace or mesh over vinyl. Soft sets go under clothes and get worn on ordinary days. Vinyl and wet look are memorable, which is exactly why they get worn rarely.
If you want the strappy look: check where the straps sit before you buy. Cage detailing across the ribs reads very differently from straps across the chest, and the photo is usually on a body that is not yours.
On care: hand wash cold, hang to dry, and wash the pair together. Washing one half more than the other is how a matched set stops matching. See all lingerie for the rest of the range.