go with the Cat 6 cable. I would also get shielded cable. Since you're in a workshop I would assume you have electric motors and such in the building? This will help cut down on any EMF (Electro-magnetic Frequency) interference.
Cat 6 cable is rated for higher and cleaner signal through put than Cat 5e. Both will carry 1.0 Gbs. Cat 6 can just do it better, especially on longer distances.