I feel a little bit guilty about leaving such a glib answer as I did before, so here's a more serious take on it:
Let's examine what it would take to unit test the service. A real unit test is an automated test that tests a single unit (in your case that would the web service without any backend systems such as databases etc.). As others have pointed out, proper unit testing of the service is probably too late in this case.
That doesn't mean that you can't use a unit testing framework (such as MSTest, xUnit.net, NUnit, etc.) to drive your service tests. Let us contrast that scenario to developing a throw-away aspx page:
- In both cases I am going to assume that the web service is already deployed, configured and running, because that would probably be the case in the aspx scenario.
- In both cases you would have to add a service reference to the test project to generate a web service proxy.
- In both cases you would have to write code that applies values to the request parameters for the web service methods.
- In both cases you need to invoke the web service operations.
So what's different?
- In the aspx scenario, you will need to collect values from form fields and assign those values to to the service method paramters. Using a testing framework, you can write those values directly. It's actually easier to write the automated test. 1-0
- In the aspx scenario, you would need to write code that takes the response data and write it to the web page. In contrast, with a testing framework, you will need to write assertions. I would claim that it's easier to write assertions, but that's a bit subjective so I'll leave this one as a tie - still 1-0
- In the automated test scenario, you will need to write many tests with different values, so that's more code you will need to write compared to the aspx option. 1-1
- With an automated test suite, you can subsequently run the automated test suite against the database many times a day with no additional effort, whereas you would need to manually input and manually verify the results in the aspx scenario for every test run. That's a huge win in testing effort. 2-1 (and that's conservative)
In conclusion, I would say that if you don't insist on doing real unit-testing in this case, but simply utilize a unit testing framework to write automated tests, you should be better off with the automated tests than with the aspx page. The development effort will be more or less the same.
For your next project, you can then see if you can use TDD from the beginning, but that's another battle.
Good luck.