Both these books are helpful. Though reaching similar conclusions, the approach taken differs markedly.
Sex, Dating and Relationships could be recommended for its opening chapter alone. A challenging and refreshing exposition of sex as an expression of the gospel sets the book's tone. Arguing that the Bible defines three 'categories' of relationship – family, marriage, and neighbours – dating relationships are placed firmly in the 'neighbour' category, leading to advocacy of 'dating friendships'. However, there is a risk of assuming a biblical example always constitutes a biblical command (which I was not fully convinced these categories do).
The Dating Dilemma is, on the surface, more supportive of modern society's view of dating and relationships; current practice is to be redeemed and improved, rather than torn up entirely. It is intensely practical, and also strong on putting God at the centre of all the things we do, encouraging us to date differently. The theological approach is quite different to Sex, Dating and Relationships, but the underlying 'answer' is not so different beneath the very different terminology.