The 2015 rmbp has a 74.9 watt hour battery according to Apple's website.
http://www.apple.com/macbook-pro/specs-retina/
That is just 5 watts shy of being twice the size of the battery in the rmb.
Essentially, for $1299 you get a retina macbook with a smaller screen, half the battery, 1 port, worse keyboard, 30% slower processor, and thinner aluminum more prone to denting/flexing.
For $200 more you get twice the battery size, magsafe charging, 2 thunderbolt ports, 2 usb 3.0 ports, hdmi out port, sd card reader, better keyboard, 30% faster processor, and thicker aluminum built like a tank.
Ironically, after buying the usb-c $79 adapter to do anything other than charge your rmb, your only $120 difference. Add in an education or military discount and your only $70 in difference.