This is probably the easiest decision you can make surrounding fenders because your bike makes it for you; it will have eyelets for braze-on mudguards or it won't.
Clip-on fenders, on the other hand, are built with brackets and straps, which attach to the fork legs, seat stays, downtube or seat post.
However, there's a saying among mechanics that you can fit mudguards onto any bike, all you need is patience and P-clips. With that in mind, if your bike doesn't have provision for bolt-on mudguards, but you want a more secure fit, you can use P-clips fit onto your fork legs or seat stay to create an eyelet - however, be sure to protect the frame with either heat-shrink, insulation tape or rubber tabs (the sort that are used to shim bike light mounts) at the point of contact, otherwise the contact will damage your paintwork.
Threading your mudguard through a rim brake calliper isn't always a simple task, and must be considered when investing in a set. Some fenders are designed to split at this point to offer full-length coverage on race bikes.