There is more to it than just tax and ni. Here are a few things to consider:-
1. Security. A ltd company has it's own identity. If something "bad" should happen when working, such as an accident where someone gets hurt. You are in a better position as long as processes have been followed and people are working safely. If you employ, I would definitely go Ltd for your peace of mind.
2. The sole trader can sell the good will to the Ltd company. This can be used over the next five years to reduce your tax liability.
3. Impressions. Some [not all by any means] commercial customers would rather deal with a Ltd company than John Smith. Even though Mr Smith does an excellent job for the same money.
4. Exit. If you want to sell at some time. The fact that your accounts are for a Ltd company and more detailed can help with the sale of the business.
5. Yes, the accountant may charge more, to be fair he is doing more work but you can negotiate with him or find a cheaper one if you wish. I have found paying more for accountancy to be a great investment. Mine is great but not cheap.