SOAP vs REST
In my work I've had occassion to use both SOAP and REST in the client and the server. SOAP is easy if you have good tools. Hard wiring a WSDL is not my thing. At the risk of committing a pun foul, I'd rather eat a bar of soap than hard code good WSDL. Fortunately, .NET makes WSDL for simple web services easy, both on the server and client end of things. And WSCF makes more complex web services easy in the .NET world.
At the same time, REST is more comfortable, especially for those without nice support tools for consuming SOAP on a plate of WSDL. A nice simple HTTP POST. A simple agreement between friends to pass X, Y, and Z data along in a simple name value pair model.
Trust or Verify
I guess in some ways it comes down to trust. Do you trust the client to submit clean data? Can you trust your server application to parse through and make safe any data that is not clean? Or would you rather automate some of that authentication via schema and the rigidity of SOAP? For me, it all depends on the circumstances.
The Illusion of SOAP and Schema
How tight are your contracts? A good lawyer will take a two page agreement and expand it to ninety pages. Not only because she wants to bill you more but because she needs to cover all the bases. Are your web service contract bases covered? Is the schema and secondary validation sufficient.
Can REST Be Secure?
This line of thought takes me to the question. Can we trust REST? Well, the short answer is no. But the longer answer is, yes, just as much as we trust SOAP. The brilliance of SOAP is the contract is carried with the data, or at least that data is transported in a container with which the contract may be validated. So is that really better? Well, the underlying truth is that someone else wrote a bunch of helper code to help us perform the first level of validation in the message--form. But what about content. Yes, schema validation can do some content validation as well, especially of the type type of validation. Beyond that, it's up to you pretty much.
Validation Bottom Line
Ultimately the value and robustness of a web service, whether you use REST for it's simplicity or SOAP for the niceties of automated tools, will be determined by the code you write to validate and execute and respond with an appropriately formed response.
Consider Your Audience
Back in my poor old days as a technical writer, I had to always keep in mind and understand my audience. It really does matter. For example, my mother would not understand a single word of this post. If you are publishing web services, you must consider who will consume them. Will they be a hodge podge of PHP, JSP, ASP, and many other forms of "server pages" technology? Or will they be hard driving Visual Studio SOAP users who would rather have the tool do the heavy lifting and eliminate the need to parse?
Give your users what they want. And to do that, you may have to give them both SOAP and REST. I guess that won't hurt us too much. After all, a hot shower is always a good combination with a good night's rest.