In either case, you get the combined benefit of N scavenging by the grass cover crop and N additions from the legume cover crop. The vegetation intercepts more raindrops before they can dislodge soil particles. Sunlight is used more efficiently because light that passes through the tall crop is captured by the low-growing crop. Adding grasses to a fall-seeded legume improves soil coverage over winter and increases the root mass to stabilize topsoil. A viny crop like vetch will climb a grass, so it can get more light and fix more N, or so it can be harvested more easily for seed.
Mixtures can complicate management, however.
Seeding rates for each component of the mix are usually lower than for sole-crop plantings, but the total seed cost may still be more. The best time to kill one crop may not be the best for another crop, so a compromise date may be used. If you use herbicides, your choices may be limited when you plant a mixture of legumes and nonlegumes. Sometimes you can end up with more residue than your equipment can handle. The benefits of a mixture will usually outweigh these disadvantages, but you need to be prepared to manage the mixture carefully to prevent problems.