We have been using CAT6 for the default in our installations for over 5 years. On smaller jobs the cost is not much. On larger jobs, it can make a difference. CAT6 has more capable bandwidth. CAT6 can accommodate speeds up to 10GbE for short distances. CAT5e can accommodate speeds up to 1Gbps. Additionally, some clients with future-proofing goals, require CAT6A for the job for longer distance 10GbE networks. While CAT6a can be great for workstations that require 10GbE, we typically recommend Fiber between MDF and IDF installations. (see Question 12 for more details).