I owned a L830 for almost a year and then gave up because of ATT update nightmare and switched to the L640XL. Both are great phones in my opinion. The L830 is a beautiful phone with the metal frame and the changeable battery cover. It has the better camera plus it has 16Gb storage which I often miss on my L640XL. 8Gb of storage is bare minimum and sometimes I have to move stuff to get another OS update. The camera on the L640XL is actually very good as well and with a registry hack I can take DNG (raw format) images as well. Another feature that the L640XL doesn't have is a dedicated camera button. The L830 has a two function camera button which is nice (focus and shoot). This is definitely a big plus if you take a lot of pictures with the phone. The L640XL doesn't have advanced audio settings either and I miss the Dolby features I had on the L830. I like the large screen on the L640XL but it is personal preference whether you like or dislike a phone of that size. The viewing angle mentioned above was pretty bad on the L830 which I don't notice as much on the L640XL.
I only know the L830 on WP8 and WP8.1 and was not able to try it with W10M so I can't comment on that. The L640XL runs without issues with W10M for me and my wife.
At retail price I'd definitely go with the L640XL. If somebody offered both phones for the same low price I'd have a hard time to decide. Not sure how long the L830 is supported with updates on W10M and further compared to the 640XL.
Dedicated camera button, more storage, better camera and the dolby audio would probably sway me towards the L830....I personally don't miss wireless charging as much as I thought I would and if I really wanted I could mod the 640XL to get that as well.