Kiwifruta: Since the palm oil incident, we've been a Whittaker's household, no intention of changing back, regardless of the price difference.
Due to the difference in the brand's block sizes, there isn't really a price difference at all.
Using the sale price that these often go for in supermarkets ($3 for Cadbury, $4 for Whittaker's), you're looking at 15c per gram for Cadbury, and 16c per gram for Whittaker's.
At RRP: 19.96c for Whittaker's vs 19.95c for Cadbury.
Cadbury isn't worth buying no matter which way you look at things.