To expand on that:
If we are talking about a particular "job" then it won't be your choice whether it's self employed or employed as buildersmate says. The Inland Revenue will effectively dictate the answer to that. Generally speaking the test os who decides when or if you work. If you are told where to be and at what time then you are an employee. If you take on a contract yourself and negotiate directly with the householder about when you will do the job then you are self employed.
If we are talking about two entirely different opportunities here though (it's not completely clear in the question) then there are pluses and minuses in both. From a tax point of view self employment is generally more advantageous than employment. You can claim for a lot more "expenses" in self employment and you have a cash flow advantage over employees in terms of when you pay any tax and NI due. However, there's no sick pay if you miss work, no holiday pay and if you are the sort who spends what you have at any given point the fact that you'll get a tax bill to pay once or twice a year (depending on how much it is) may prove more of a disadvantage then an advantage in terms of cashflow. Also if you are self employed you need to keep your own records, and either do your own tax returns or emply an accountant to do them for you.
There's no right or wrong answer. Assuming both options are generally available to you then it's up to you to decide which best suits your needs.