Not much. http://www.trustedreviews.com/opinions/iphone-6s-vs-iphone-6
Personaly, dont think it's worth it. BUT THEY FIXED THE BENDING OF THE PHONE AND THEY HAVE ALUMINUM 7000 series on 6s( harder to damage ).
iPhone 6S: 7.1mm thick, 143g, series 7000 anodised aluminium back, Space Grey/Silver/Gold/Rose Gold
iPhone 6: 6.9mm thick, 129g, series 6300 anodised aluminium back, Space Grey/Silver/Gold
iPhone 6S: 4.7-inch, 1334 x 750 IPS LCD, 326 ppi, 504 nits brightness, 3D Touch
iPhone 6: 4.7-inch, 1334 x 750 IPS LCD, 326 ppi, 504 nits brightness
iPhone 6S: Apple A9 64-bit dual-core 1.84GHz, PowerVR GT7600 GPU, 2GB RAM
iPhone 6: Apple A8 64-bit dual-core 1.4GHz, PowerVR GX6450 GPU, 1GB RAM
iPhone 6S: 12-megapixel, phase detection, dual LED 'True Tone' flash, 4K video, 5-megapixel front camera
iPhone 6: 8-megapixel, phase detection, dual LED 'True Tone' flash, 1080p video, 1.2-megapixel front camera
iPhone 6S: 1715 mAh non-removable battery (bullshit m8)
iPhone 6: 1810 mAh non-removable battery