Both are two-fin setups. Both go fast. But they feel different under your feet, and they suit different boards and different surfing.
Twin fins
Narrower base, taller profile (14cm height, 11.5cm base). They are responsive and loose. The board pivots quickly and feels skatey on the rail. Good for shortboards, fish, and anything where you want speed with manoeuvrability.
The narrower base means less drive than a keel, but more freedom to move the tail around.
Keel fins
Wider base, shorter profile (11.5cm height, 14cm base). The wide base generates drive - the board accelerates through turns instead of sliding. The shorter height keeps things loose enough to feel fun, but there is more hold than a standard twin.
Keels suit retro fish, mini Simmons, and boards with flatter rockers. If your board was designed to go fast in a straight line with gentle rail-to-rail movement, keels will feel right.
Which should you choose?
If you want loose and skatey: twin fins.
If you want drive and hold with that twin-fin speed: keel fins.
If you are not sure, go keel. They are more forgiving and work on a wider range of boards.