Traditionally, manual has been the preferred option, for two main reasons. First, if you pass on a manual car, you are also entitled to drive an automatic, but if you pass in an automatic, you will have to take the test again if you want to drive a manual. Second, most cars (in this country, anyway) are manual drive.
However, with an automatic there is no need to worry about the clutch or gears, making it easier and quicker to learn, and being restricted to only driving automatics after your test is far less of a drawback these days with most manufacturers offering automatic versions of all their models, many of them small and relatively economical.
So, if you are struggling with clutch control or gear changing, or in a hurry to pass your test, an automatic can be a sensible alternative.