One-piece raincoat has good waterproof performance, can completely fit the body and avoid rainwater infiltration. However, they may make people feel stuffy, especially during long-term cycling. In addition, the one-piece raincoat may be inconvenient to put on and take off, especially when the hands are wet by rain.
The coat and trousers of the split raincoat can be separated, which is more convenient to put on and take off. In addition, split raincoats are usually lighter and more comfortable than one-piece raincoats, and are more suitable for long-term riding. However, split raincoats may not be as waterproof as one-piece raincoats, because there may be more gaps to be waterproof.
To sum up, if you need to ride in rainy days and have high waterproof requirements, then a one-piece raincoat may be a better choice. However, if you pay more attention to comfort and convenience, then a split raincoat may be more suitable for you.