Usually, the most up to date information is best, I'm sure if you need to see an old edition afterwards, you can go and find one at your university library. Having said that, Cotton & Wilkinson contains a wide variety of subjects and if the new sections that have been added are completely irrelevant to your research, and the ones that were removed are intimately involved, then obviously you're going to have to make an informed decision as to which one to get. Other than the major changes of new chapters, remember that new editions mean that more information has been found about the other topics covered by the book and possible corrections of errors.