i have a 6d and a 70d, in good light there is not much iq difference. the 6d is of course much better at high iso. paired with the new 16-35is you have the ability to shoot landscapes at small apertures in low light. if you are normally shooting in good light it doesn't matter much. below is a comparison between the 6d + 16-35 f/4 (16mm) and the 70d + sigma 8-16 (10mm) i have taken about 100 comparison shots just to see what the difference is and in good light you have to look really close to see anything and then sometimes it's not clear. don't get caught up thinking there is some massive iq difference between the formats that just jumps off the screen at you.