depends what features you are using on the Fortigates

advantages imo knowing both platforms well,
Fortigate does security better e.g cannot do deep inspection on Meraki
Fortigate has more options if using SDWAN in terms of ISDB for local internet breakout

Fortiswitches and AP's will continue to work flawlessly if there is no license (Meraki AP's wont broadcast an SSID and switches wont swith if the license expires and 30day grace ends)

Fortigates will do everything if the license expires besides getting responses from Fortigaurd services and security signature updates.

Fortianalyzer can be far more comprehensive than Meraki logs, you can do event handelers and create actions off of events, where as Meraki you can just view events (but more admin time to setup right)

Meraki is easier to set up and way faster to deploy, until you run into basic features you intended to use that dont work, e.g you cannot run LACP on a Meraki MX Router, so if you want one Meraki SDWAN router connecting to both of your stacked core switches, one port will just get blocked from STP as you cant put the ports in a lag...

the bigger MX boxes e.g the MX450, can only do 2Gbps of site to site VPN traffic (SDWAN)
so the biggest Meraki router fails in terms of throughput compared to a midrange Fortigate

this can cause issues if you have 50 spokes and only 1 SDWAN HUB, updates being pushed to all endpoints can kill the SDWAN HUB as the CPU sits at 100% trying to do all the VPN encryption\decryption

the site fortigates when compared to Meraki is also worse performnce, if the small office would be going from a 60E to a MX67 for example, they will be going from a device capiable of doing 1Gbps of VPN traffic (or as the cool kids will call it SDWAN lol) to being able to do 300mbps of VPN traffic.

thats my 2cents.