You will need to define a clear use-case to get a specific answer to your question. Unless we have a detailled use-case here, I'd like to answer with general advice on that:
SOAP vs REST
SOAP and REST are two ways of accessing web-services.
SOAP uses XML for its requests. REST relies on HTTP requests via URL and is therefore really lightweight compared to SOAP.
(Dis-)Advantages of SOAP:
- well defined web-service
- has pre-built standards (SOAPv1, SOAPv2, SOAPv2 WS-I)
- works well in enterprise environments (due to standards)
- some tools can be automated by using the WSDL
- heavyweight compared to REST
(Dis-)Advantages of REST:
- easier to use
- more flexible
- smaller learning curve
- effictient/lightweight compared to SOAP
- no defined web-service structure (no WSDL)
Magento SOAP API vs REST API
Be aware that not all features are implemented for the REST API.
I created a small performance test-script which does nothing more than 10x10 login and endSession requests via SOAP API v1. No specific action was performed.
1 request took between 0,9 and 1,4 seconds with my local webserver.
To compare with rest: Once your application is authorized (where you need to authorize your application once via clicking "authorize"), it takes only a few miliseconds to be ready to perform your request after you have obtained a token and a secret: 2.0-3.0 * 10^-5 seconds locally.
For conducting a performance test which could finally answer you question, you need to define the requirements.
The following would be interesting:
- Do they login once and perform multiple actions or does each of them login for only one action?
- What kind of requests do they perform? Adding/updating products, placing orders,... ?
- SOAP API v1, SOAP API v2 or SOAP v2 WSI vs REST?
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