Size, and also, if you care for unlocked devices, the 550 does the job. $80 is the price locked to AT&T. You can unlock it, but your device would still be locked to when/if AT&T decides to provide updates. Do I need to mention the tale of the 830, or not? I have also heard talk about the 635 getting updates but the GoPhone variant did not.
I have the T-mobile variant of the 640, and I like it a lot but I seriously doubt that T-mobile will roll out Win 10 Mobile, considering how fast they dropped the phone. I really like the idea of Microsoft truly having the power of updates.
So that's my $0.02. I think that's a great price for a phone that is truly unlocked and I'm interested in playing with it, but I seem to be the only oddball who has a 640 and feels this way. I have adjusted to having a 5" phone (came from the 520/521 series) but I miss the slightly smaller phone factor. It packs nice specs for what it is.
Edit: It does a great job of improving and essentially cleaning up the 4xx and 5xx series.