I described using using DHCP between the user computers and the SonicWall and using DHCP between the SonicWall and the 2Wire.
You responded with
"I guess my main concern is that I don't want 2 DHCP servers dishing out addresses simultaneously. With that said, I am left wondering which device I should set as the default DHCP server."
Maybe you are thinking of setting this up completely differently than I have done. What I have successfully repeatedly and always done with small offices and small routers is
The user computers connect to the LAN ports on the router. Between those computers and router is either DHCP or static addressing. If there is DHCP then there is a DHCP server handling that.
On the other side of the router or firewall device, connected to the WAN port on that device, is a completely different world. If there is DHCP between the WAN port on the router and the modem that has, as far as I know and have seen again and again, nothing to do with what is done on the LAN side of the router. What happens on the far side of the router has nothing more to do with what happens on the near side than than if say Google happens to have DHCP running somewhere, that is invisible and isolated from your LAN side of your network.
Now if you are putting your network together differently, where all your local computers and your SonicWall all hang off the LAN ports on the 2Wire then all these can see all these and you only want one DHCP server. I have never done that because I want nothing to get to the local network without having gone through inspection by the firewall in the router.
Please, try it before you are standing in front of the customer. Get three different brands of routers, each with a different 192.168.x.y admin/setup address, daisy chain all three, set up DHCP between each pair in the chain and verify that this works, just to prove this is correct or to come up with any evidence that there is anything wrong in this.