When I specced these two up on a spreadsheet, the 4/4 came just under £40k and the +4 just over, with a couple of grand between.
You're paying for the privilege of speccing a car to your exact requirements. Is it worth it? who knows, but I could be tempted. Then again, the first (and only significant) hit of depreciation is in the first year, so give up the thrill of speccing your baby and you'll save maybe £5k, maybe more.
If you want a comparison, look at SC Elise prices. £35k new, and £25k after 3 years with continuous depreciation thereafter.