Depends on data complexity
If data is linear or with just 1 or 2 levels of tree-nodes, either XML or JSON would be okay with OE10x
OE11/12 is obviously way forward as JSON is industry wide recommended for REST services which XML is for SOAP
OE10 with complex JSON will be time consuming in fixing bugs and custom handling of the data. OE10 sometimes fails with new data-types and their representations e.g. Double Quotes inside Double Quotes or as peter said handling of "curly brackets". There might be more.
Take a careful decision by looking into data complexities and possibilities to upgrade OE. You may also think on creating your own JSON parser which is again time-consuming in long-run of getting matured.
I am bit surprised to see both solutions (XML/JSON) being available together. Are you sure these are not linked with 2 different end-points which facilitate 2 different services like server-to-server vs REST API calls kind of. If so, you may need to decide for solution from business/operational level.