Canon EOS 600D has an accelerometer, The accelerometer is a sensor used to measure the orientation or vertical and horizontal positioning of the device. The most basic application of this is the device knowing when to alternate the view of the screen depending on how you are holding it. In addition this feature allows app developers to utilize this feature in games and other programs.
Also 600D has Faster video autofocus due to a phase detection autofocus when shooting movies, A phase detection autofocus is much faster than a contrast detection autofocus and allows sharper movies.
But, Canon EOS 700D has a microphone in. Its a microphone port allows connecting external high-end or specialized microphones.EOS 700D camera as AF tracking. With AF tracking, once you choose the subject and press the shutter release part way down, as the subject moves, the autofocus will follow it. No more out of focus shots.
Canon EOS 700D is 4x higher maximum light sensitivity. Higher ISO levels capture more light, avoiding blur or allowing to shoot in low light situations. However, higher ISO settings usually result in lower quality images.
EOS 700D also 35.14% faster shooting than EOS 600D at highest resolution with AF as JPEG format. Fast continuous shooting is useful for catching action shots.
Canon EOS 700D is a better choice than 600D, but is up to you and your budget. :p
Canon EOS 600D vs Canon EOS 700D