Hi, it basically depends on the customer's requirement because you can use the existing MX68CWs for SD-WAN solution without buying MX100. Each of the MXes models has different capacities in terms of the number of users, routes, VPNs...etc that it can handle so it mainly the deciding factory if you need to upgrade or use an MX100.
Here is the link to Meraki MX products,
https://meraki.cisco.com/lib/pdf/meraki_datasheet_mx.pdf
hope this helps>
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