Host DHCP services on server or firewall/router?

The only real pro's and con's are the ones you've already addressed, which is continuity of the network. I usually put DHCP on the primary server (DC,DNS) so that DNS records can be automatically updated if/when a client's DHCP lease expires and it is issued a new IP address. It would take additional configuration to accomplish this task using the Sonicwall as the DHCP server.

The other side of the argument is that keeping DHCP and DNS settings on the SonicWall will allow continuity of client devices in regards to internet access. But with the server down, unless you only use hosted services, all they're going to do on the internet is mess around.

It really is up to you, but I hunted down some more opinions about the matter for you. Feel free to take a look at this and this. Basically, it's up to you, and there are caveats to each side. I personally recommend keeping it on the server.

I'm in agreement with sybreon and JohnThePro as well as the original post, but I thought I would add a few more thoughts.

If the client PC's are on a domain, I would not suggest using the ISP (or external) DNS servers for clients - if for any reason a client starts using the external DNS servers, these will have no knowledge of your internal structure so will not have the relevant A records for your servers and printers, etc. Also active directory creates a few SRV records that are needed for AD to correctly work. It is possible to recreate the entries say on a *nix box running bind, but this requires manual work. A second domain controller can automatically update all these records as needed.

Also I like using windows DHCP servers as they are highly configurable, most other DHCP servers do not provide all of the functionality available using windows DHCP.

Another benefit is when things start to go wrong, you will have less different things to look at.