I currently have the Late-2013 15" Pro, with 2.0 GHz processor, Iris Pro, 8GB RAM and 256GB SSD. I'm looking to upgrade and know that I will be needing a larger hard drive, so my options appear to be either to purchase the top end 13" Pro with the 512GB SSD (£1203.60), or to spend an extra £486 and buy the top end 15" Pro for £1,699.20. My first doubt is whether the 13" would actually be a downgrade, as I mainly use my laptop for photo editing (hence the need for a larger hard drive) and it could be potentially slower in Lightroom than my current Mac. However, I'm also wondering if it's really worth spending the extra £486 for the 15", as I'm not sure if it's much faster in real world use than my current Mac. Would I be better off just buying a portable hard drive, or are they pretty slow?