sorry, scottish language mate apologies.
The sonicwall devices are pretty solid tbh, Ive not configured one recently but if it has the option to run both connections simultaneously and also failover if one drops then thats the way to go.
Also force your mail to go out of the correct IP address or it will be rejected as spam by alot of servers, failing that just add a secondary MX record to the domain name for the other IP address.