The MDR-ZX700 is a consumer grade headphone, while the MDR 7506 is a professional grade headphone. If you want a flat (neutral) sound, spend the extra 23 euros and go for the 7506. If you don't want a flat sound, save your 23 euros and go for the MDR ZX700. What are you going to use your headphones for?
I'm going to use them for music on my j3, gaming, movies etc. so inside and outside use.