Mikrotik is a better router and will (with the 36 core network processor) definitely move more traffic with less latency.
However, Mikrotik is NOT a UTM appliance like Sonicwall/PFsense/Untangle. Its OS (RouterOS) does have a relatively decent firewall (basically ACL-based with some simple dynamic rules). But it doesn't support any kind of signature-based rules, has very limited logging and simplistic management.
Depending on your objectives it might do just fine. You'll be impressed by it as a router. But unless you were using just the most rudimentary features of the Sonicwall you will be disappointed with its capabilities as a UTM.
BTW - I use Mikrotik routers all over my various networks and really like them. I am not a detractor. Just want to be sure you are looking at it for the right job.