For me, this is how I would compare them:
830 Pros:
- Size of phone (form factor)
- Physical camera button
- Wireless charging battery covers included
- Super Sensitive Touch for use with gloves
- Double the internal storage of 640 XL
830 cons:
- AT&T version did not get necessary firmware update during the time I had it
- Netflix and Bluetooth issues due to lack of firmware update
- Capacitive buttons weren't sensitive enough/Didn't work at all with through film screen protector
640 XL pros:
- Larger screen size
- On-screen buttons are more touch responsive than capacitive buttons.
- Higher camera resolution
- Longer battery life
640 XL cons:
- On-screen buttons take some getting used to
- No physical camera button
- No wireless charging without modding the phone
- Size makes it more difficult to use one-handed
I've been using the 640 XL for about 7 months after carrying the 830 for about 4 months. The biggest reason I abandoned the 830 is due to the lack of firmware updates because I had the AT&T version. I'd have kept the 830 if AT&T would have given any indication of the future supported updates or if I would have had the unlocked version of the phone.