IMO, buy it in Spain. I believe the price will be the same in France and Spain but you'll get a bigger VAT refund. Go to Global Blue website to calculate the exact VAT Refund. I wish I knew this before I made a purchase in Paris. Happy shopping!
Store prices will be the same. In terms of a VAT refund (all prices displayed are inclusive of VAT in Europe) then the VAT rate in France is 20% and it's 21% in Spain. Whilst you don't get all of the VAT back, there's not much in it to be honest.