We've extensively tested both full-length and 3/4 waterproof pants over the years. In our experience 3/4's provide the best trade-off between functionality and performance in most conditions - especially for mountain biking, bikepacking and touring.
- They are easy to whip on and off over your shoes.
- Repel rain from the most exposed areas - your bum and thighs.
- Avoid 'chain suck' at your ankles.
- Minimise overheating when pedalling hard.
However full-length over trou are sometimes best - notably for commuting when you want to keep your long pants dry, or when touring to really cold and/or wet places.