"hey Cortana" is a Lumia feature, where Cortana can be activated hands free by simply saying the words "hey Cortana", instead of tapping the search button, but it is exclusive to devices with at least an snapdragon 800 processor leaving the 830 out... Also the 830 may have less megapixels, but alot of people say the 830 produces the same results as the 1520,930, or even 1020 just with less sharpness, due to less megapixels..
A lot of people will agree the 830,930 & 1520 cameras overall out due the 1020 camera, since its only edge is a massive amount of pixels creating a larger image, great for cropping.
The three I mentioned are in general the top Lumia Devices (1520,930,830) right now and apparently have improved camera tech over the 1020. I myself haven't owned a 1020, but I've played around with it in store before, and all my sayings are simply conclusions drawn from various reviews and camera comparisons around the web... So what I'm saying to you is pretty much just a gather of info, more than experience