Let me first say that neither will increase the security of your network greatly, though static is preferred.

That said, dynamic is considered a little less secure because if someone does manage to connect to your network, all they have to do is request an IP address and your dhcp server will happily give it out.

If someone connected to your static network, they'd have to guess the range of the ip addresses in order to use your network.

Now, in a cleverly designed network, an intruder would never be able to guess a valid ip address because the network admin filtered the MAC addresses to correspond to IP addresses. That makes it a lot harder for the intruder to guess the MAC address AND the IP corresponding to that MAC.

Unfortunately, there are a lot of network admins that think turning off the DHCP server _alone_ will increase security. But a quick google search could show you some of the more popular routers and their default IP addresses, router passwords, etc. This is why it's wise to change your networks address, submask, and filter filter filter as much as possible.

Of course, I'm not a Cisco (or any other kind of) expert... just my $0.02