Well, I am able to test an API method using JSON request in Fiddler and can view the response correctly. Also web services can be tested in SoapUI. So what's the difference between them?
The answer is similar to real world examples, both Motor Cycle (2 wheeler) and Car (4 wheeler) provides approximately same speed and can carry a person from one place to another, so why two (or in actuality more than two) types of vehicles are there and that too with further classifications (different brand designing different vehicles) i.e. because each vehicle offer some unique features which other may not offer and they both differ in some respect like Mileage, Size, Cost, Capacity, Comfort, Safety etc.
Similarly, in software market there are many tools available for functional, performance, security etc. testing (some paid and others free). Each tool provides some set of features (or extra features) which are not provided by other tool (or if provided by others they may not be in same way). Some tools are costly, easy to use, feature enriched, supports lot of protocols, can also be used for performance etc. while others not. In the same way there are certain limitations and feature gaps which are provided by SoapUI, but not by Fiddler and vice-a-versa.
Fiddler is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software, communications protocol development and education. Fiddler captures HTTP and HTTPS traffic data between browsers and servers. One of the most powerful capabilities Fiddler offers is capture of traffic from almost any device (iOS, Android, Windows, Mac and so on). Personally, I have used and saw people using Fiddler for the purpose of debugging and capturing traffic for Performance tuning and resolving issues. It will capture all the network traffic (although you can control that).
While, SoapUI cross-platform Functional Testing solution. With an easy-to-use graphical interface, and enterprise-class features, SoapUI allows you to easily and rapidly create and execute automated functional, regression, compliance, and load tests. In a single test environment, SoapUI provides complete test coverage and supports all the standard protocols and technologies. So, this tool offers many such features not provided by Fiddler and it works with service calls interactively. It captures only that traffic/calls which you tell it to capture.
We have used it mainly for SOAP based WebService(s) Functional and Performance testing and it's very good at that. It is available in both Free and Licensed editions and I found this more easy to use than Fiddler.
But, it all depends upon your needs and requirement, if your requirement and end-to-end testing is achieved by using Fiddler and you already have experience in using it than you should go for it, but if you are comfortable with SoapUI and want to expand your testing area than use that tool.
Fiddler is a proxy that lets you view all network traffic, like WireShark.
SoapUI is a full fledged testing tool.
My recommendation is to try POSTman. It's a free API testing tool, you can save your test into collections if you need to reuse them. I use POSTman for my initial testing and then use a CSV + PowerShell (using the HttpClient from .net 4.5+) combo to make my regression type tests.