My local bicycle mechanic advised me to get mudguards (such as the ones in Image 1) to extend the life of my drivetrain and protect it from spray in the winter. If I have £20 to spend on either a chainguard (see image 2) or a front fender, would I be better off with a chainguard? Would this be lighter and provide better protection for the drivetrain vs front-wheel-spray? Image 2 shows how a chainguard on a bike (3-9 derailleur) could block both spray which would be stopped by a front fender (green lines) and spray which would slip past underneath a front fender (blue lines).
Are there any chainguards which would divert mud flowing along the underside of the down tube, so it drips to the ground before it can flow onwards and build up around the axle for the pedals? (Otherwise I guess I could accomplish this by DIY-fixing a mudflap to hang down from the down tube.)
NB, I would still use a rear mudguard regardless.