Speed, speed, speed and speed.
Did I mention speed?
Latency is another factor. If the people on the network use database for example, every query will be a bit slower than on a wired network. If their applications do lots of queries the latency adds up.
Security is another problem, even if you encrypt your access point, the clients have the password. "A secret shared is a secret revealed"
Only go for Wireless if:
You must have mobility in the office, meaning everyone does not have a fixed desk, etc.
You cannot afford installing a wired network, it is more expensive than a wireless network, and a lot more work to install.
But if you can, go for fixed wire. Gigabit if you can
Edit: Adding a Wireless network as a backup, or extra network, is always a valid option, but it is still a security risk.