Any will is valid as long as it is properly witnessed and signed. So (subject to those conditions), you can prepare your will using a form from any decent stationers, by using an online service or by scribbling it out on the back of an envelope.
However, as the former MD of a will-preparation company, I would caution against using any form of will-preparation service which only establishes what is to happen if just one of you dies. The first question I would have asked you is "So what should happen to your estate if you're both killed in a car crash?".
Chris