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Having used both Lumia 1520 and Lumia 830 I can say they are fine devices. On paper either is superior to a 640 or a 640 XL.
If a 6" screen device is fine for your use, then by all means the 1520 is unsurpassed in the hardware and features offered for Windows Phone and will not bet bested until a new Flagship is released under Win 10 Mo. And it will have "Hey Cortana" support. That 2GB RAM is also nice when juggling heavy tasks.
If 830 looks nice to you I would say it is very worthwhile. Both the 830 and 1520 will give you great service and have many of the advanced photo elements you are used to from 1020 like DNG. Check the shootout thread linked in my signature for many, many samples. In some ways the 830 may be a better camera in general than the 1520 due to the f/2.2 aperture which you are used to from the 1020. It seems very balanced in terms of light gathering vs depth of field.
If photography is not your primary concern over modern build, go with 640 XL as it is more upscale in Camera than 640 (but not as well as 830 or 1520).
Given your field, I'd drop 640 off the list unless your highest two priorities are modern date of build/design and smaller screen.
