Pro's and con's a lot of the time depend on peoples views of limited companies.
Main advantage to you is your limited liability. As a sole trader you are totally liable for any problems that occur as a result of what you do, whereas as a limited company it is the company which is liable and as an individual you have no liability (as long as there is no negligence or wrongful trading involved).
For that reason, some people are wary of small limited companies, but that is simply a perception.
Cons are that your accountancy bills will be higher as a limited company as there is legislation about what accounts must be prepared, whereas as a sole trader the legislation is much lower (if not even non-existent). Also, as a new limited company you will have no credit record/history so gaining credit terms with suppliers etc. will be harder to achieve.
Why have your clients asked you to be a limited company?