The CreateResonse method is more suited to cases where you want to control the response header details
- it creates a properly formed httpresponse structure with all the appropriate metadata and the like as part of the response wrapper.
- and then allows you to manipulate those headers etc before passing the response back - You can change headers or content type etc
- and also to completely control the content negotiation process.

The DTO method on the other hand - does not let you have this level of control - its i guess closer to the bone in terms of what it passes.

For example - if you use the DTO method there is no easy way to report a HttpStatusCode.NotFound or HttpStatusCode.BadRequest
- sure yes you could maybe do something like set the idkey of the dto to -1 or some such
- but thats not really ideal if ajax is the main consumer of your data.

If you use the CreateResponse method - you can easily pass back Request.CreateResponse(HttpStatusCode.NotFound);
which ajax will automatically know how to respond to

So as such - to a large extent it comes down to what is consuming your data - if its ajax then you should be handling the content response appropriately.
If you are maybe doing some other forms of process to process communication then DTO may be suited - as it is somewhat lighter on bandwidth.

Have a look at the response you get from each sort of request using something like fiddler - that would give you a better idea of the bandwidth/content that is travelling around under the hood - beside you actual serialized request data.