It would seem that the step up. Macbook m5/512 would be the same price (or less) as MBP i5/256. Instead of $100 more.
Not to mention 2 usb 3.1/Thunderbolt 2 ports vs 1 usb 3.1, support for more/bigger external monitors.......
It's not cheap being small. With apologies to Kermit.