I'm from that area (near Caernarfon). I was most upset at not being able to afford Welsh slate. The house is a semi so I did not have "full" control of the job. The cost was shared between us. You cant really mix synthetic and proper slate so it was one of the other.
Best quality Welsh slate used with pressure treated battens, copper nails AND fitted by someone that knows what they are doing --- will last a 100 years -- if maintained. Loose slates fixed, missing/broken ones replaced. Don't let anyone up on the roof without proper ladders etc! A missing or loose slate that catches the wind can cause the whole roof to peel off.
Note my use of the term "best quality" - not all slate would last 100 years!