Why?
Steel is harder than Aluminum, right? With a bolt carrier, it's bare steel against anodized aluminum. The Bolt carrier itself doesn't need any special coating. Heck, it used to be chrome plated (AR-15 and M16 Air Force models) and they decided that was overkill and went to parkerizing. We've got carriers from the 60's that have been used in training weapons almost continually since that time without wearing out, you AIN'T going to wear out yours. If you have a TiN barrel extension, TiN bolt, carrier, cam pin, and gas tube, the only appreciable difference you'll EVER see is one of... COLOR!
Now then, TiN is not easy to apply as it's vapor deposited so it tends not to get into tight areas or holes well. Mirror-bright Chrome is the way I would go if I had the choice between Chrome or Titanium Nitride. Actually, TiCN or Titanium Carbo-Nitride (Black) is the way I'd go. Again, it'd have to be mirror finish. Me... I chose Parkerized 'cause of the cost/benefit... it isn't worth the extra money for looks.