Mudguards come in all shapes and sizes, and the more wheel coverage they offer, the more spray they will prevent. A full-length fender will not only protect you and the rider sitting on your wheel from tyre gunk but also your frame and bottom bracket will be pelted by less salt-infused road grit.
Look for fenders that have some contouring or a rounded shape, if they are too flat they won't offer all that much protection.
Even full-length mudguard coverage doesn't necessarily prevent that arc of spray that comes from a fast-spinning wheel, fitting a flap (an Ass Saver performs well) to the end of your mudguards is a great way to offer absolute coverage, and it'll probably help you make friends on the winter club run.