Depreciation is a bit of a 'dark art' at times but this seems very strange. If you sell it privately, it's unlikely to make any difference what pack you have or don't have as buyers will just see leather sports seats. It can be different if you are trading in or selling to a dealer as used prices are affected by optional extras - so the designer pack may be seen as being more desirable (it also comes with anthracite headlining and a different wood trim) than simply listing it as having leather seats. We suggest you go with your instinct and get the seats you want - it's very unlikely to make £1500 difference if you sell.