Microsoft DHCP only for reserved addresses

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.

The Microsoft DHCP server has no concept of a list of MAC addresses that are outside one of the configured DHCP scopes. I'm also not aware of any way to get the Microsoft DHCP server to return an IP address outside of the scope in which the DHCP relay agent's GIADDR address falls. I don't think you're going to be able to replicate what you had w/ the Microsoft DHCP Server.

As an aside: Before you consider handing-out IP addresses to public clients using a Microsoft DHCP server you may want to get a clear statement from Microsoft as to whether or not these devices need CALs. Microsoft has made contradictory statements about this. To be "safe" I always use a non-Microsoft DHCP server for public subnets.

Edit:

You're absolutely right that a DHCP superscope will allow IP addresses to be assigned to clients outside the subnet of the GIADDR that the relay agent passes. It's not a feature I've ever used in production but, even so, I feel like a bit of an ass for not thinking of it.