I run both of these firewalls in my company and they are very different. As an example, Meraki can't do source natting which is why I use a Sophos in my headquarters. Intrusion detection on Meraki is either on or off for the entire firewall. Sophos lets you enable or disable IDS on a per interface basis. Sophos does SSL client VPN which is why I use it for my VPN clients. However, for all of my retail locations I use Meraki because it's so easy to manage a large amount of remote locations.
Meraki might be better for you, or it might not be. It depends on your requirements. Meraki really shines with lots of remote locations or simple networks, but it falls apart pretty quickly when you have complex networking needs.
Also, please note that the Sophos SG series is being replaced by the XG series. If you are going to Sophos, I would look at the XG series.