buy an old iphone 6 (3yrs) when iphone 8 is up to date

The iPhone 6 and iPhone 6 Plus are smartphones designed and marketed by Apple Inc. The devices are part of the iPhone series and were announced on September 9, 2014, and released on September 19, 2014.

The iPhone 6 and iPhone 6 Plus jointly serve as successors to the iPhone 5S and were themselves replaced as flagship devices of the iPhone series by the iPhone 6S and iPhone 6S Plus on September 9, 2015.

The iPhone 6 and 6 Plus were discontinued in most countries on September 7, 2016 when Apple announced the iPhone 7 and iPhone 7 Plus.

The iPhone 6 and iPhone 6 Plus include larger 4.7 inches and 5.5 inches (120 and 140 mm) displays, a faster processor, upgraded cameras, improved LTE and Wi-Fi connectivity and support for a near field communications-based mobile payments offering.

Both branded as "Retina HD Display" and "ion-strengthened", the iPhone 6 display is 4.7 inches in size with a 16:9 resolution of 1334x750 (326 ppi, minus one row of pixels),
iPhone 6 Plus includes a 5.5-inch 1920x1080 (1080p) display (401 PPI).
The iPhone 6 features a 6.91 Wh (1810 mAh) battery, while the iPhone 6 Plus features an 11.1 Wh (2915 mAh) battery.  improved battery life over the 5s.

Both models include an Apple A8 system-on-chip, and an M8 motion co-processor—an update of the M7 chip from the iPhone 5s. The primary difference between the M8 and the original M7 coprocessor is that the M8 also includes a barometer to measure altitude changes. Phil Schiller touted that the A8 chip would provide, in comparison to the 5s, a 25% increase in CPU performance, a 50% increase in graphics performance, and less heat output. Early hands-on reports suggested that the A8's GPU performance might indeed break away from previous generations doubling of performance at each yearly release.

The expanded LTE connectivity on the iPhone 6 and iPhone 6 Plus is improved to LTE Advanced, with support for over 20 LTE bands (7 more than the iPhone 5s), for up to 150 Mbit/s download  (iphone 5, 5S only 100Mbit/s download) speed, and VoLTE support. 
https://en.wikipedia.org/wiki/Voice_over_LTE
Le 25 novembre 2015, Bouygues Telecom annonce le lancement de son service VoLTE, dans un premier temps réservé aux clients possédant un Samsung Galaxy S6 ou S6 edge, devenant ainsi le premier opérateur français à proposer ce service11. Le 9 décembre 2015, Bouygues Telecom étend son service VoLTE aux iPhone 6 et 6+ d'Apple équipés de IOS 9.2 ou de versions ultérieures.

Le 23 février 2016, Orange annonce le début du déploiement progressif de la VoLTE associé à la voix sur Wi-Fi (VoWiFi) sur les réseaux mobiles 4G de ses filiales européennes.


Wi-Fi performance has been improved with support for 802.11ac specifications, providing speeds up to 433.0581 Mbit/s—which is up to 3 times faster than 802.11n, along with Wi-Fi Calling support where available. 
The iPhone 6 and iPhone 6 Plus add support for near-field communications (NFC). It is initially used exclusively for Apple Pay—a new mobile payments system which allows users to store their credit cards in Passbook for use with online payments and retail purchases over NFC. iOS 11 will provide support for uses of near-field communications besides Apple Pay.

The iPhone 6's rear-facing camera now has the ability to shoot 1080p video at either 30 or 60 frames per second and slow-motion video at either 120 or 240 frames per second. The camera also includes phase detection autofocus. It can also record. The iPhone 6 Plus camera is nearly identical, but also includes optical image stabilization. 
l'iPhone 6 propose une simple stabilisation numérique tandis que l'iPhone 6 Plus utilise un stabilisateur optique plus performant. 
The front-facing camera was also updated with a new sensor and f/2.2 aperture, along with support for burst and HDR modes.

When first released, the iPhone 6 and iPhone 6 Plus were supplied pre-loaded with iOS 8.
To improve the usability of the devices' larger screens, an additional "Reachability" gesture was added; double-tapping the Home button will slide the top half of the screen's contents down to the bottom half of the screen. This function allows users to reach buttons located near the top of the screen, such as a "Back" button in the top-left corner.

technical data

Model iphone 6 . : A1549, A1586, A1589
Compatible networks GSM, CDMA, 3G, EVDO, HSPA+, 4G, LTE
Dimensions iphone6:
H 138.1 mm (5.44 in)
W 67.0 mm (2.64 in)

iphone 6: 129 g 

Apple A8

CPU 1.4 GHz dual-core 64-bit ARMv8-A "Typhoon"

GPU PowerVR Series 6 GX6450 (quad-core)

Memory 1 GB LPDDR3 RAM

Storage 16, 32, 64, 128 GB

Battery 6: 3.82 V 6.91 W·h (1,810 mA·h) Lithium polymer battery

Data inputs

Multi-touch touchscreen display, triple microphone, Apple M8 motion coprocessor, 3-axis gyroscope, 3-axis accelerometer, digital compass, iBeacon, proximity sensor, ambient light sensor, Touch ID fingerprint reader, barometer

Rear camera 8 MP with 1.5 focus pixels, True Tone Flash, autofocus, IR filter, burst mode, f/2.2 aperture, 1080p HD video recording (30 fps or 60 fps), slow-motion video (720p 120 fps or 240 fps), timelapse, panorama (up to 43 megapixels), facial recognition, stills from video, auto-HDR

Digital image stabilization

Front camera 1.2 MP (1280×960 px max.), 720p video recording (30 fps), burst mode, f/2.2 aperture, exposure control, face detection, auto-HDR

Sound Mono speaker, 3.5 mm stereo audio jack