IMac retina display 2015 vs Imac 2013

I want to buy a new IMAC , but i don't know what to choose,

the retina display had amazing screen

but the 2013 working faster then retina display

my current using is, PhpStorm, and Xcode

which one working faster?

as i see the retina has low cpu according to 2013 model

Apple 27" iMac with Retina 5K Display (Mid 2015)

27" Retina 5K IPS Display

5120 x 2880 Native Resolution

3.3 GHz Intel Core i5 (Haswell)

8GB of 1600 MHz DDR3 RAM

1TB 7200 rpm Hard Drive

AMD Radeon M290 GPU (2GB GDDR5)

802.11ac Wi-Fi, Bluetooth 4.0

2x Thunderbolt 2 and 4x USB 3.0 Ports

FaceTime HD Camera, Dual Mics, Speakers

Mac OS X 10.10 Yosemite

VS

Apple 27" iMac Desktop Computer (Late 2013)

3.4GHz Intel Core i5 Quad-Core (Haswell)

8GB of 1600Hz DDR3 RAM

1TB 7200rpm Hard Drive

NVIDIA GeForce GTX 775M (2GB)

27" LED-Backlit IPS Display

2560 x 1440 Native Resolution

802.11ac/a/b/g/n Wi-Fi, Bluetooth 4.0

Thunderbolt, USB 3.0, SDXC Card Reader

720p FaceTime HD Camera, Dual Mics

Mac OS X 10.10 Yosemite