Wifi hacking has more scene. So maybe this is because your client thinks is safer... but it is not. Anything can be hacked if you don't take the appropiate security measures. Both of them can be victim of DoS too.
Advantages of using wifi
- Higher range, so if the factory is large, less hardware will be needed to cover all, son less money I guess.
- More compatible hardware.
Advantages of using bluetooth
- I don't know! maybe they are less people focused on bluetooth hacking, so less tools.
Anyway, I think (personal opinion) A wifi network can be a good idea if you do well the configuration. Some standard security advices:
- Disable WPS
- Use WPA2 AES encryption, is the strongest of the standard available for now.
- Use certificates or if not possible, use very long non-existing in dictionaries passwords using lowercase, uppercase, symbols and numbers.
- Use if possible Radius instead of PSK (if possible).
- Use "clients isolation" if possible. Only if not needed to connect from clients to clients.
- Your wifi network should be in a different network subrange with port filtering to assure only accessing to the minimum required services.
- If your money is ok, use hardware WIPS (wireless intrustion prevention systems) to try to prevent DoS. This is not usual, is not cheap.
With that standard security measures, you have a good wireless networks. Anyway, always everything can be hacked, you know :)
Regarding the key exchange on wireless networks and on bluetooth...
Bluetooth security is limited to key matching. In wireless networks the security standards have been raised with inclusion of new versions from the old-easy-to-hack WEP until complex key exchanges or certificate validation like on EAP-TLS.
Some other opinions supporting this on this article.
So finally my opinion too is Wireless is more secure, or at least, has more security mechanisms.