If you're really worried about it, I would use the older bike. As it is, I can't ride my nice bike to the University... I have to take my older one. As for a lock, I'd probably use more than one kind and make sure they aren't any cheap ones that will be cut through without incident. I have a Kevlar reinforced Specialized cable lock that is about an inch diameter and additional U-lock if I'm feeling exceptionally worried. Have you asked your employer or security about the possibility of bringing the bike in to your building?