VueScan is more logical and easier to use, and as such I found myself more prepared to experiment to achieve the best results. This is already a major point in VueScan's favour because, in the past, it was often only the oblique interface presented by Dimage Scan Utility that limited my ambitions when scanning difficult slides or negatives. The temptation with Dimage Scan was always to create a scan that looked 'good enough' and then work on it in Photoshop, whereas using VueScan I find I can get extremely close to the desired end result using the driver along - and without pulling my hair out.
Functionality and ease of use aside, if you've recently upgraded to Windows Vista and your beloved scanner will no longer work, VueScan will save you upgrading your expensive hardware unnecessarily. Whatever your priorities, for as little as 20 GBP for the standard version VueScan is extraordinarily good value.